tricep push downs with supinated grip
 

I read a suggestion on here to supinate as well as pronate your grip when you do these.

However when I do them with the supinated grip my left arm wants to internally rotate. I really lightened the load with this exercise so I can get the correct ROM without pain. Also on DB skullcrushers my left arm wants to abduct. I'm guessing its the same muscle doing this. I do a shitload of mobility work with my yoga practice so I'm guessing its a weak something or other.

Any ideas as to what could be causing this?

I used a vv bar when I do tricep work with a suprinated grip. It allows the wrist to be internally rotated a bit, which helps as I have forearm and elbow issues.

Are you using a straight bar (--) to do these? I wouldn't.