No addons at all. It glitched after the 10th joust.
Exited, re-ran WoW repair.
Tested again, no addons at all. It glitched after the 17th joust.
So, multiple vehicle quests across several zones glitch shortly after I try "converting" someone to nUI, and it's just a bloody coincidence.
Sorry to waste your time.
I think I need to order another case of Monster Rehab....