I've seen several unexpected increases on the order of 10 points over the last couple of weeks. (I don't remember the exact dates.) My guess was gradual undoing of prior mass-downvoting, but a Mystery Mass Upvoter is certainly another possibility.
[EDITED to add ...] A possible variant of the Mystery Mass Upvoter hypothesis: we have a Mystery Small-Mass Upvoter, who is upvoting old posts in Main (maybe because s/he is new here and reading through old material). But that only works if everyone affected has old posts in Main, which I don't think is the case.
Hypothesis: we are the subjects of an experiment.
I seem to recall recent instances of a mysterious mass downvoter that produced several threads of people complaining / trying to figure out what could be done.
What if someone is doing the same thing, but with upvotes, to look for bias in community reactions?
Or they're just trolling. Whichever.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.