Umpire Brian O'Nora Abruptly Ran Off the Field in the Middle of An Inning

The Chicago Cubs and Colorado Rockies are playing tonight at Coors Field and a supremely strange thing happened in the top of the third inning. Home plate umpire Brian O'Nora halted the game and proceeded to jog into the clubhouse. He didn't return.
Check this out:
Home plate ump Brian O'Nora jogged into the home dugout in the top of the 3rd and will not return to the ballgame.
— Billy Krumb (@ClubhouseCancer) September 12, 2023
Here's quick video look of Brian jogging off the field.@Cubs #GoCubsGo @Rockies pic.twitter.com/4jmgkS6dbb
There's been no explanation of what happened but, as stated, O'Nora did not return and fellow umpire Tom Hanahan changed into protective gear and got behind home plate.
We're just assuming O'Nora suddenly wasn't feeling well and had to get off the field, but I've never seen play halted in the middle of an inning for that. Usually an umpire will leave the game between innings unless something physical happens -- like getting hit by a ball, etc.
Speculation online was that O'Nora just had to use the bathroom badly. While that's a funny angle, it wouldn't explain why he didn't return. We should get some clarification after the game finishes, but this was really odd.
We have a lot of fun with MLB umpires around these parts, but we hope O'Nora is OK.