Why Do Moments of Mindlessness Occur?

You're off to a good start, in that you are aware of periods when mindfulness is absent. When does this awareness occur? Much later, just after, or during? Try to catch it sooner. Be like a detective, and trace mindlessness back to it's source. See if you can remember what conditioned it. The sooner you catch it, the easier the trace will be.

Everything is conditioned. If we can observe mindlessness non-judgmentaly (or anything else) as they arise, the conditions that give rise will become known. Know what gives rise to it's coming, and what gives rise to it's disappearance. Know what it is like during mindfulness, and what it is like without. Observe keenly, as though you were expected to report on it later. Develop a keen interest in what's actually happening, and experience it fully for yourself.

It is not enough to simply hold the object with your attention, one must "rub" into the object, so to speak, to understand it moment to moment. Observe how it changes and disappears. The conditions that give rise to mindlessness will become obvious. Often, it is the hindrances - clinging, aversion, sloth, restlessness, and doubt. The hindrances are simply the mind out of balance. Rebalance yourself in the middle way and mindfulness will become more effortless.

For example, becoming mindless while browsing the internet is probably both sloth, aversion, and craving - to lazy to do something else, desiring a good story to make you laugh, aversion towards being bored, etc.

In short, the five hindrances disrupt mindfulness.

https://en.m.wikipedia.org/wiki/Five_hindrances

Also related to the five hindrances are the general defilements (kilesas), which also hinders mindfulness.

I am also wondering whether merely paying close attention to lapses in attentiveness and to distracted moments can ultimately lead to the elimination of such habitual behaviours.

It’s a good start. However, one needs to purposely commit to the entire eightfold noble path in order to completely eliminate those behaviors.

In the following talk, it is described that the five hindrances, namely, sensual desire, ill will, sloth and torpor, restlessness and remorse, and doubt, do not just affect a person in meditation. It also affects a person at other times. The five hindrances obsess the mind and enslave the mind. They also appear habitually.

The speaker says that people who commit acts of crime like raping their own daughter, are strongly deluded and enslaved by the hindrances.

Only after attaining and mastering the first jhana, do the five hindrances reduce to a very low level that is not significant enough to be considered hindrances any more. After attaining the fourth jhana, the five hindrances are nearly completely eliminated.

The speaker says that a person who has overcome the five hindrances is capable of yoniso manasikara (focused attention or careful attention). With this, a person could attain stream entry simply by listening to the true Dhamma (teachings) attentively and reflecting on it. This is based on SN 55.5.

So, why do moments of mindlessness occur? It's due to the presence of the five hindrances.

In the YouTube video entitled "Characteristics of a Sotapanna" at timestamp 25:52, Ven. Dhammavuddho Mahathera stated (with some paraphrasing):

The third condition, this word - yoniso manasikara, which I translate as focused attention. It is also known as careful attention. This word means that when you listen to the Dhamma, you are focused on listening to the Dhamma. In other words, at that time, you don't have the five hindrances. If that is so, then you can understand the Dhamma and attain stream entry.

So who are the people who do not have the five hindrances? In the suttas, it is stated by the Buddha that as long as a person has not attained piti and sukha which are secluded from unwholesome states, which are secluded from sensual pleasures, the five hindrances will obsess him and obsess him habitually.

So, there are two types of persons who do not have the five hindrances. The first is the one who has attained the first jhana (i.e. he who has attained piti and sukha). When a person has attained the first jhana, he has eliminated the five hindrances and the Buddha says that the five hindrances no longer obsess that person habitually. ...

So there are two conditions if a person has the hindrances. Firstly, it obsesses your mind, it enslaves your mind. Secondly, it is habitual, it is very often there. That's why when somebody rapes his daughter or rapes his granddaughter, it's because he cannot control these hindrances. His hindrances overwhelms his mind, obsesses the mind. After a person attains the first jhana, the Buddha says that the five hindrances are eliminated.

A lot of people don't believe it because they don't really understand the hindrances. If I ask you what are the hindrances, a lot of people will say sensual desire, ill will, sloth and torpor, restlessness and worry, and doubt. But these are not the five hindrances. These are called the five hindrances only when they obsess your mind, when they enslave your mind and they recur habitually. Only then they are called the five hindrances.

When a person attains the first jhana, these five hindrances reduce to a low level. They are still there but they are not called hindrances any more. It's just as if you walk through a jungle, you may encounter seven foot high long grass obstructing you. It's a hindrance because when you try to go through it, you cannot see in front of you. Secondly, it cuts or injures your skin. So, it's a big hindrance. But if I cut down that long grass to 12 inches, it is no more a hindrance. But it is still observable.

So, when a person attains the first jhana, it's as if the seven foot high long grass has been cut down to 12 inches. When he attains the second jhana, the 12 inches becomes 8 inches. When he attains the third jhana, it becomes 4 inches. When he attains the fourth jhana, it becomes one millimeter. So there is a big difference between the first three jhanas and the fourth jhana.

That is why it is said that once a person attains the first jhana, the hindrances are (nearly) eliminated, meaning generally (nearly) eliminated for good.