Not sure about the exact conditions but I think it has to do with mousing over multiple close together map icons. Sometimes one of them will get stuck. I have disabled all of my map and rare related addons and narrowed it down to Silver Dragon . I switched to Rarescanner which has the drop items within the item's info panel rather than as small single item panels. Solved it for me until Silver Dragon has a fix.

Technically they're multi-item panels! It's just most rares only have a single interesting drop. :D

They shouldn't be capable of sticking around when moving between close pins now.