I understand the flat call but I think with Ace face some sort of raise is required, particularly for this eventuality because if an ace does flop, you have represented it and can be more sure that any show of strength is hands that have you dominated and not vice versa.
There's not enough info to guess what hands these might have and you have no reads on them. I can't understand trips going all in (but you never know) so my best guess would be pocket pair v. A10 or A8. UTG could have struck gold with his chips but my guess is that URG+1 has made two pair.