To "pick" means to kill a plane already engaged with a friendly. It is derived from "cherrypick" as in "to select what one considers to be best or most desirable, profitable, etc. from a number of options."
One can see how the derogatory term "picker" came about in Aces High, as in "a skill-less tard who flies in furballs and kills players who are already fighting someone else," (which ignores the fact that it takes considerable skill, SA, and good gunnery.) So a players good score or rank or whatever is explained away by saying "he's just a picker."
When you're dove on and killed by a higher con, regardless of whether you see him or not, it's a bounce, not a pick. Perhaps it's the association of the word "cherry-pick" and being up on a ladder (or in an actual cherry-picker attached to a vehicle) that makes players mistakenly associate being killed by a higher enemy with the term "pick."