Why does this happen? See video:
Also: sometimes the automap gets bugged out and shows you some random area when entering a new level, anyone know anything about this? Sometimes it reveals the stairs to the next level.
I guess it might be worth mentioning here that in the hardcore category, using townportals on level 5, 9 and 13 to cross over walls and lava rivers or difficult boss packs etc. can be really helpful, assuming you have a up to town nearby and perhaps need to go to town anyways.