i also had this phenomen some times, if its a yellow or magenta item thats realy annoing
iam not sure what causes this event, but it seems like the enemyposition is the reason. Allways when i saw that, multiple items were dropped. So maybe they dont have enough space near the wall and some "walk" into the wrong direction while dropping. I guess the wall is only impassable for characters and monsters, but not for items.