Antiboss farm

What is this exactly and how does it work? I notice that in HM if I kill a boss the first time it drops great items; lock picks, elite tomes, greens, and or passage scroll. Second time i kill him, the drop diminish to rampage, hero, adventure scrolls. Third time is basically whites and after that nothing. Then I start again maybe 10 hours later and the drops are restored. Is this anti boss farm code tied to a boss, to a character or to an area?