Apparently he got hurt before the 619 sequence
Update from PWInsider:
A video of the complete bout has made it's way online, which provides a greater picture of what happened before Aguayo collapsed. All four men are battling in the ring when Aguayo is nailed with a missile dropkick. He takes a bump that results in him flopping over and landing on his stomach. He returns to his feet and takes a head scissors from Mysterio, bumping outside to the floor, twisting his body while running to take the move and go through the ropes. Aguayo lands on the outside and returns to the ring and is dropkicked, resulting in him landing against the ropes in the traditional position to take the 619.
It appears Aguayo may have suffered a spinal trauma while taking the bump to the outside of the ring, not realized it (possibly due to the adrenaline of performing) and worsened the injury taking the dropkick after he returned to the ring. Once he hits the ropes, Aguayo is completely out and it appears that since he landed in the normal position to take the 619, that greatly added to the confusion as initially one would think he was just selling.