Looks like he had to fly through the weather he wanted to go around. Maybe some freakish downdraft.
Perhaps clear air turbulence. The precipitation didn’t look too bad. Whatever happened, it seemed to be sudden.
TWC said there is a 140 mph jet stream, the air is turbulent. Being denied the request for a change of course sounds like an issue.
Go to 4:00 in this recording and you’ll here them warned of light to heavy rain straight ahead. A couple of minutes later they request to go around it.
http://archive-server.liveatc.net/kiah/KIAH-App-Feb-23-2019-1830Z.mp3