I seem to recall something regarding this in one of the previous patches, where they implemented that if a certain mob is being farmed and die fast, they will respawn faster to help the players get theire quest goal achieved.
This is called dynamic respawn and they implemented it with 2.1 or 2.2...

But this can be dangerous too. It has happen to me more than once, that I entered an area and killed some quest-mobs. I didn't notice that someone just left the same area as i entered. So I thought, there are only a few mobs and charged straigt into them. After a few seconds, the dynamic respawn popped back up and had its fun killing me.