I've thought about this, especially recently now that you can drop any Storage Unit in Alpha 4, regardless of inventory state...
It's not currently 100% random, but I believe it should be. In fact, right now you can somewhat control what drops--dropped items are always taken from the end of your inventory list, so you can control what drops via sort commands.
Inventory manipulation is not detailed enough to make controlling this very feasible without being fiddly, so it's something I don't want to add. The result may seem sort of "game-y", but I like that there's less control of that aspect, a sort of price to pay for dropping a unit haphazardly, especially now that storage units are so incredibly protected from destruction--huge integrity, low coverage (also against logic), immune to rejection, saboteurs, critical hits, and most other negative effects... If you must have the control in a given situation, spend the little extra time to drop them manually (which also costs game time, so it has a real cost associated with it).
I can see how this matters even more for those of you running around with massive inventories, now that we have a pretty extreme "go naked" command. Though really, if you're going to truly drop almost everything I would assume this isn't a situation where you're carrying a lot of stuff to begin with--or at least these two situations will rarely coincide.