I've heard about this and saw them talking about it on a hunting show, I think they were hunting colorado.

What I've heard is the whitetail is a more aggressive deer, so a hot muley doe may get bred by a whitetail buck as he's more likely to chase off a muley buck.

They had mentioned something about whitetails being more active at night whereas muleys are more active throughout the day? I think is what they suggested. Or maybe something like a whitetail is more likely to sit still vs. jump and run like a muley.