One guy forgets to lock his skis, his buddy has to go out anyway to meet someone else and offers to lock the first guy's up while he's out there, locks up the wrong pair, and the first guy forgets that the other guy was going to lock the skis up at all and so is not surprised to find his skis out in the rack, unlocked, and skis off...leaving some poor third party's skis locked up in the rack. You have to have the second party in this picture, or the explanation for some third guy's skis getting locked up becomes insanely complicated. The owner of the skis is going to know whether he's locking up his own skis or not - and one way or another, he's going to know they were locked whether they were his or not. So if he comes out and finds his skis there, unlocked, he's got to wonder whose skis it was that he locked up, and - presumably - retrieve his lock, thus leaving the third party's skis unlocked, which did not happen. The alternative is that the guy gets drunk while skiing and is drunk while locking up the third guy's skis, which is how he doesn't come to realize he's locking up someone else's skis, and remains drunk (or gets drunker) while he's taking a break, which is how he comes to forget that he locked up a pair of skis to start with. Another alternative is that someone thought that they'd lock up a stranger's skis, count on the owner of those skis not noticing that their skis are locked up, or not being willing or able to get assistance from the mountain employees for this, and then just abandoning the skis until a later point in time when the thief can sneak back, release the skis from their bondage, and steal them.
The first option just requires crossed lines of communication and an average level of forgetfulness. The second requires someone with profoundly impaired awareness or major mental disturbance. The third requires someone who is profoundly stupid, but with a low cunning. While I think that you could find people in the last two categories if you look, the first type of person is someone I encounter in every single lift line. Not paying attention, talking at cross-purposes with other people in the group, and creating coordination problems for everyone in the group by doing so. Can't swing a cat without hitting one of these people on the hill, weekends.