Lots of reasons. Valid reasons for the occasional rundown might be multi-tablers engaged in big pots on other tables, but this should be infrequent and not every hand. If they're doing it all the time then they're either playing too many tables, or their intention is likely some sort of bubble tactic in tourneys, or just trying to tilt opponents.
You see it pretty often in satellites on the bubble where there is a micro stack at the table -- kind of an implicit "collusion" where everyone wants their entire table to cash so to help protect a tiny stack from blinding out, every player will run the time out.