I think there was a lot of Clinton-Republican voting because Clinton voters didn't expect Trump to win and felt comfortable voting for (e.g.) Leonard Lance and Pete Roskam as a check on her. When that reality went unrealized, it was time to put a check on Trump instead.
American politics has this weird schizophrenic obsession with checks-and-balances that is frankly awful.
I agree, I think that's exactly what happened. Swinging voters in Clinton-Republican districts assumed Clinton would win so they should vote Republican down-ballot. I don't think that's a totally crazy posture for a moderate voter who would genuinely prefer a compromise result to take, either, although I think relatively few voters are actually moderate on issues instead of holding non-moderate views on different issues in both directions.
