Thanks for the information and help everyone!
Yes, it's totally on me for not asking. I was more focused on getting on that flight (regardless of cabin) since it was about 90 minutes from departure, and if I didn't get on that flight, it would get home really late an overnight. There ended up being one available F seat on the flight, though I don't know if it was for sale or not when I was being rebooked.
The reason wasn't disclosed, so I didn't have a lot to go on if I wanted to push for F anyway. The inbound aircraft was bound for IAH on Saturday but diverted due to Harvey, I presume, and it had been there the whole weekend. It was supposed to fly from BTR to my departure airport on Monday afternoon, but was delayed. UA could have easily claimed weather at BTR as the cause, though it could have been crew for all I know.
In any case, I thought UA was generous to let me rebook for no charge when the original delay was only about an hour (though it ended up being almost two ultimately). But I'll remember to ask for F, even on OAL, in the future now.
IIRC this fare was was a Y-up fare. I really wish UA would fix that issue. If people are paying cash for F, it should be treated as F in all situations.