Based on my observations, spam is the #1 cause of old threads being revived.
#2 is new people to the board who were googling for something specific, found something here, joined and commented without noticing the date. Or sometimes they do notice, but believe it's better to comment on an old thread rather than starting a new one. (I personally think it's better to start a new one, because a lot of people read the first post, not the most recent one.)
#3 - Sometimes at the bottom of a thread, there will be a box with suggested threads related to the thread you're currently viewing. There are no date limits on these suggestions, and I think that's another possible way the old threads gain new comments.