Threads get locked for a number of reasons - the fastest is probably when a thread starts heading political (as in blue/red), or culturally 'flash point'
Other reasons:
Impossible to keep it on subject (pretty much every thread, but they usually die a natural death before the mods take action)
Impossible to keep it civil - two or more posters start a back and forth and it just isn't worth the mods time.
Too long - starts eating into board bandwidth - hence monthly recruiting threads.
Too far off topic, particularly in season.
Generally being a source of banned comments even if the thread topic is OK
And of course my favorite:
Moderators decide to be completely arbitrary and mess with our heads!