I doubt that the pond turned over. It sounds like the pig manure, as you have realized, created a problem of over fertility and a dissolved oxygen crash occurred. Larger fish always die first with only some very small fish, green sunfish in this case, surviving along the shoreline.
If the runoff situation hasn't been corrected, expect a repeat performance.
I see nothing wrong with green sunfish. However, if you have too many of them, that evidently doesn't meet the farmers goals. Bass will eliminate a lot of them. However, sooner or later, expect the bass to overspawn in an 8/10 acre pond.
