Their feet do get tangled and stepped on because their lead foot is postioned funny. When they stand directly in front of eachother in their fighting stance, its almost as if they're standing beside eachother shoulder to shoulder while trying to face eachother at the same time. That shit is uncomfortable in person, and looks waaay more uncomfortable watching on television. Hell, the fighters are uncomfortable half the time, so yes stepping on eachother's foot is one of the many weird things that will happen in there. Real hard to explain, but easier to demonstrate.
The real question is, why was Pac's foot so available to be stepped on in the first place? If his footwork was so "excellent" (according to the pom-pom-wavers), then why were his feet getting stepped on? If Marquez knew in his mind that he couldn't afford to be out of position, then why would he risk it trying to step on Pac's foot? These questions aren't anything against the two, but it goes to show you that it happens when guys are right handers and southpaws.