Is there a way to find out why, specifically, my billboard ad was rejected? If it's something that can be fixed and resubmitted, I'd definitely like to know.

You could try filing a ticket. I don't know who handles billboards or if they're able to give out more information but that's gonna be the most reliable way to get ahold of staff about something like that

i had a rejected billboard a while ago and didnt realize this was an option, so thanks for posting, i just submitted a ticket as well