He was out on his feet and then his feet buckled under him. You can see in the video how his right foot wobbles right after the punch. The punch was not hard but it landed well and hurt him. He would have been better of if it landed flash to give him some horizontal velocity to fall - perhaps his knees wouldn't buckle in that case and he could beat the count and continue.
Looking at just how badly hurt the punch had him, it would be safe to assume that VITLAY probably would have had Solis out of there by the second or third round anyway even if Solis had fallen square on his ass.
He got hit and that took his legs. While wobbly he might have injured his leg. Thats what it looked to me.
Yes he SHOULD lose some weight over the long term but why the **** would he need to drain himself before a fight? he has no weight to make! duh!
It's probably more than one factor of the things you've listed. He lunged in, got clipped by a shot that grazed his head. Then panicked when Vitali charged, and back stepped wrongly landing on his ankle...