For what its worth, downvotes aren't undoable here, and I've fat fingered the button a few times when meaning to upvote on touch screen devices. If you're getting random unexplained single downvotes, its possible someone else did that and didn't realise. I'd imagine such issues explain a significant number of cases where people go from 1 to 0 without anything downvote worthy.

It'd be nice if I could disable the downvote button while on my phone, as I've given almost as many unintentional downvotes as intentional ones. Or at least have a minute to change the vote after submitting.

(Obviously this doesn't explain bulk downvotes or anything of that kind)