On July 21, there were 92572 unanswered questions on Math.SE according to the
unanswered questions statistic (and not the
not answered statistic). On July 22, there were 94797 unanswered questions — a difference of over 2000. This is visible in the Unanswered Question Tracker in the Crusader's Chatroom.
We didn't get 2000 more questions in a day. I see perhaps five possibilities:
Stack Exchange changed its fundamental definition of an unanswered question in some way, perhaps just on some sites (like perhaps Trinity+Math, or SO+Math, or some other age/volume based way). Under the new definition, approximately 2000 more questions became unanswered.
A 2000+ vote user deleted their account for some reason.
A very large amount of voting fraud was caught and automatically reversed by either the Celestial Thaumaturges (CommTeam) above or the automatic script.
As unlikely as it is, a very large number of users just happened to downvote/remove upvotes/unaccept answers in the same day.
Alternately, either the API from which the Unanswered Question Tracker from lied (though rather consistently for a very long time) and/or the Unanswered Question Tracker is simply buggy (although it is correct now, and no change has been made to it).
This is mysterious to me, and I do not understand what has happened.