It also happens, when I meet an orc patrol and decided to [f]ight with them. So, it is not related with dungeon generation, rather than entering into new place.... or it is related with monster generation/placement code.
PS! In this game, I decided to avoid dungeons and explore the wilderness...mostly, to avoid this bug. Unfortunately, the bug catch me... and there is no way to avoid this bug in 0.2.3.
I hope, this one gets fixed soon, since this is really annoying bug... my best chars have been ruined because of this nasty bug.