Does anyone else seeing a sudden increase in unnecessary pubsub callbacks from AppEngine-Google since ~7:50 UTC today?
Do you need few more examples?
All hub's vitals look normal. There is a slight drop in the number of feeds processed by the hub per second, but it's not as dramatic as on your graph.Is it possible that many of your past subscriptions expired at about the same time?
The only other explanation I have right now is that there was a changes in one of the big feed providers (such as blogger or feedburner), but that's just a guess.