Diff is supposed to go down to adjust to 10 min block time
I have no idea why hash rate would go down from 8400 to 7900. Bitmain testing batch 2?
I could make a guess that they put a lot of units online over the weekend for testing. There would be no other reason why the difficulty would keep climbing over late Saturday and all of Sunday. The last of the first batch shipments landed at peoples doors on Friday. Even if a handful were Saturday, no way that justified the levels of difficulty increase that we saw over the weekend. It was parabolic.
That does sound like a reasonable explanation... We're below 1000 Sia a day now... When I started it last Wednesday we were at ~4k/day. Fortunately I just gambled on a single A3 for fun ;-)
edit:
All this just makes me love my Baikal Giant more... Cheap, low power and multi-algo!