Why was Zabrze renamed Hindenburg O.S in 1915?

The decision to rename the place from Zabrze to Hindenburg in 1914/15 was not unique to this "big village".

WP: Belomorskoje:
Belomorskoje (russisch Беломорское, deutsch Hindenburg, bis 1918 Groß Friedrichsgraben I)

was renamed in 1918 for exactly he same reason, nationalistic proud and fervour, making it necessary to rename Zabrze once more from simply "Hindenburg" to "Hindenburg/Oberschlesien" to avoid mixups. The whole place only got the full rights of a real city in 1922. Although older, genuine Hindenburg place names might have been additional factors complicating addresses:

Hindenburg, Saxony-Anhalt, Hindenburg in Templin, Lindenhagen in Nordwestuckermark (formerly Hindenburg until 1949).

Just looking at the pronunciation cannot reveal the sole reason. It was of Slavic foundation but became thoroughly German dominated for the last centuries before the name change. Silesian Germans were not always as nationalist and quite capable of pronouncing at least some words of their neighbours near correctly, if they weren't bilingual… A difficult Slavic name might well be a small contributing factor, given that even prominent proponents of change might fall back to the old default:

I was in Szczecin because of Winston Churchill, who had no trouble with the pronunciation when, in a speech in Fulton, Missouri, in March 1946, he uttered the prophetic words: "From Stettin in the Baltic to Trieste in the Adriatic…" He used the German name Stettin because the city had been part of Prussia for 225 years.

Although that reasoning is not well attested.

Szczecin, Poland. This Polish city on the Oder River has a rich history, starting in the 8th century when it became a Pomeranian stronghold. It is perhaps better known by its previous name - "Stettin", and featured in Winston Churchill's famous Iron Curtain speech: "From Stettin in the Baltic…"

Apparently he stuck to the old name despite accepting Polish rule over the town. This change or the resistance to it might be politically enforced or not. Quite often it seems less than a statement to such end but rather benign (Stettino).

More surprising seems that a location with this history and geography apparently did not have a name in the 'other' language, like Köln/Cologne, Danzig/Gdansk, Brünn/Brno, Eger/Cheb, Lemberg/Lwiw/Lvov/Lwów), Dresden/Drežďany/Drážďany/Drezno)…

Name changes were not totally unheard of for German cities either (Long list on Wikipedia, though not all 'pure' name changes). From a more international perspective, List of city name changes can provide a glimpse into how common renaming was and is. A very close matching example in time and intention from a non-totalitarian country is a city that changed its name in 1893/4 as a tribute to Marshal Floriano Peixoto from Desterro to Florianpolis.

Through history, many places got renamed, often by totalitarian, ultra-nationalistic or communist regimes. Most of this renaming happened past 1918 under the influence of Wilsonian self-determination, where people would eliminate traces of "foreign" imperialism.

While indeed quite many places were 'renamed' and many for somehow political reasons, most of these cities changing their names shortly after 1918 already had an equivalent local dialect version of that same name that only took precedence over the 'old' name officially. But that is true in the other temporal direction as well. Before 1918 we have all kinds of name changes, from Byzantium/Constantinople/Istanbul to Saarlouis/Sarre-Libre/Saarlouis(/Saarlautern/Saarlouis). Sometimes in favour of a person, sometimes in defiance. Whether a person was chosen as a name patron and whether the person was already dead was not a generalisable criterion to rely any reasoning on. Only now have people realised that honouring a living person too often backfires when choosing a new name for anything permanently. (Think of Stalingrad, Hitler-Platz,… )

But when you compare other things named after Paul von Hindenburg, like bridges:

Hindenburgbrücke (Begriffsklärung):
Nach dem Reichspräsidenten Paul von Hindenburg wurden mehrere Brücken Hindenburgbrücke genannt:
Hindenburgbrücke, zerstörte Eisenbahnbrücke bei Bingen über den Rhein (1915–1945)
Hindenburgbrücke (Mosel), zerstörte Eisenbahnbrücke bei Igel über die Mosel (1912–1945)
Hindenburgbrücke (Hamburg), Brücke der Hindenburgstraße über die Alster

Darüber hinaus gibt es mehrere Brücken, die ehemals den Namen Hindenburgbrücke getragen haben:
Berliner Brücke in Halle (Saale) (bis 1945)
Bösebrücke in Berlin (1916−1948)
Brücke der Jugend über die Seltenrein in Löbau
Deutzer Brücke in Köln (1935−1945)
Ernst-Walz-Brücke in Heidelberg
Jerusalembrücke (Südbrücke) in Magdeburg (1927–1945)
Jungbuschbrücke in Mannheim (Name zwischen 1933 und 1945)
Kallhardtbrücke, Straßenbrücke über die Nagold in Pforzheim (1928–1946)
Landauerbrücke über das Elsterbecken in Leipzig (bis 1945)
Most Tolerancji in Głogów (Polen)
Mosty Warszawskie, Brücke der Matthiasstraße über die Alte Oder und den Schiffahrtskanal in Breslau

or damms like Hindenburgdamm or towers, especially in the East (Hindenburgturm), or just houses (Haus Hindenburg, Villa Hindenburg) you might start to see a certain pattern.

The most notable predecessor for this pattern in Germany was of course Bismarck. Bismarck (Begriffsklärung), who had apples, fish, pastries, trains and trees etc named after him and most prominently some memorials like Bismarck towers built in his memory.

Some more examples of places/towns renamed after persons in Germany, not before 1500 and not after 1918: Borsigwalde, Scharmützelhütte->Ferdinandshof, Buchhorn->Friedrichshafen (1811), Friedrichskoog, Georgsdorf, Malbergen->Georgsmarienhütte (1857), Leopoldshöhe (1841), Schröck->Leopoldshafen (1833), Ludwigshafen (1852), Holzhof->Maxdorf (1819), Quilicz/Quilitz-> Neuhardenberg (1814), Selbelang/Bardelebenschen Meierei-> Paulinenaue (1833), Heppens/Neuende-> Wilhelmshaven (1869).

Wilhelmshaven was of course named after the future emperor Wilhelm I of Prussia.

Back to Zabrze itself:
According to the official newspaper – Zabrzer (Hindenburger) Kreisblatt – the town just wanted the new name, and got it "approved by the highest authorities" on March 1, 1915. No official reasoning given there.
Some revisionist sources (Ostpreusissche Allgemeine (1980), p11.) the inhabitants grew tired of a way too Polish sounding name for the town, and just wanted something more germanic. This assertion is allegedly based on the book: Josef Pollok (Ed): "Die Geschichte der Stadt Hindenburg OS", 1979 (excerpts online).
This contrasts

Before and during the world war, the German authorities had made a concerted effort to germanize place names, most famously renaming Zabrze as Hindenburg in 1915.
_{(T. K. Wilson: "Frontiers of Violence: Conflict and Identity in Ulster and Upper Silesia, 1918–1922", Oxford University Press: Oxford, New York, 2010, p189.)}

My general rule of thumb has been to try to give place names in the form they occur in the original sources consulted. Occasionally, this leads to minor anomalies – i.e. Polish usage always called Zabrze ‘Zabrze’ but pro-Germans used both ‘Zabrze’ and ‘Hindenburg’.
_{(Tim Wilson: "Fatal violence in Upper Silesia, 1918–1922", in: James Bjork, Tomasz Kamusella, Tim Wilson and Anna Novikov (Eds): "Creating Nationality in Central Europe, 1880–1950. Modernity, violence and (be)longing in Upper Silesia", Routledge: London, New York, 2016, p53–84.)}

Looking at the latest statistical developments for the region from 1890–1913:

Kreis Zabrze: 2,4% 8,7 % bilinguals
1890: Poles 23.0% – Germans 72.7%
1913: Poles 40% – Germans 51.0%
_{(Paul Weber: "Die Polen in Oberschlesien. Eine statistische Untersuchung", Springer-Verlag: Berlin, Heidelberg, 1914.)}