Just to be clear: This is not a bug, but the way the game engine handles enemy respawns. Dungeons, much like any other area, are reset after a certain amount of time (typically 12 in-game hours). So if you spend too much time raiding a dungeon, the reset is triggered at some point, but will only take effect after a reload.
In your case, I assume that you quicksaved after spending more then 12 hours inside the dungeon, so upon reloading all the enemies and traps respawned.
My advice: Keep an eye on the clock and break your dungeon run into several smaller ones if it takes too long. I know it's tempting in casual mode to try to clear the dungeon in one go, as you always can reload after dying, but this might bring you into trouble as seen here.