I thought El Perro seemed to have punched himself out early.
I look at it as Kirkland punched Angulo out early.
Angulo may have been a bit winded after his flurry of punches, but james landed this NASTY left to the body that took his air out imo, and that's what made him first stumble back. Then of course james finished him off at the end of the round.
In round 3 angulo recovered and threw more punches than kirkland, he landed a couple hard, clean ass shots on james in that round but kirkland just walked through them. Re-watching that round, I can see how a judge could give it to Angulo (he threw more). Then in the next rounds james was just imposing his will on angulo, he was just too determined, too much of a beast.
Angulo is known for his great stamina, he's the type of guy that breaks down his opponents with volume punching, never slowing down. So I don't think stamina was an issue, other than perhaps at the end of round 1. James was just in beast mode