score:15
It's more likely that there was a problem with the seat.
This could be, but is not limited to (and I've seen all of these):
As such, until they get a chance to service it, they move you.
Presumably because of the different stresses the plane goes under during acceleration, that they moved you. If you seat broke mid-flight, you could stroll around and sit somewhere else. If it happens during take-off, they can't even get up to help you (Well they shouldn't).
It's not like there's an "unsafe" seat on every flight - you don't have to try and remember to avoid say, 37A, or something. It's just that for that flight, they probably had a problem with one.