@.access:
The idea is the person overdone him/herself and passed out quickly along with the others. At least that's where the possibility if it being Rebecca is coming from.
As for myself, I am hoping it was someone from outside and the one getting up is simply a victim who managed to get up and will be the winner of this block.
I am hoping for Manshelly as the attacker, just to point it out (Doffy ordered her to fasten the tournament once he noticed Luffy left the colosseum). More like a wishful dream than a possibility though.
A winner being decided by outside interference is the worst outcome since it makes the winner look weak. Oda needs to set the seeds for the crowd to start respecting Rebecca and to do that she needs to win legitly and with her own power.
Sabo is dressed in Luffy's outfit. So everyone else including Doflamingo still thinks Luffy is still fighting in the tournament.