My guess is that was preplanned, and it ended up looking so completely out of place because Trump didn't go negative at all. Everything was positive, so Pelosi ends up looking even more like a bitter old witch than usual because she got vicious over a speech that wasn't confrontational at all.
What Trump did last night reminded me of that trick when you get in a pushing match with someone, then suddenly give way and they fall down because there was no more resistance, and they've lost their balance. He baited them with leaks that this was going to be his big revenge for impeachment, and that he'd blast them. Then he comes out and gives a very positive, forward looking speech, and they were left completely off balance. I imagine they're still trying to figure out today exactly how they were caught so off-guard, and how to recover.