Downloops will be the result of pulling the front hand on the bar. Its pretty common that most people end up pulling one hand or another - although it is often the back hand.
I suspect you may be doing this (skip to 2:04 as I can't work out how to embed the time shift):
In my case, when I'm not too powered, I tend to try and sheet the bar in when popping in order to get a bit of extra height. Its the combination of sheeting in whilst steering upwind for me that means I unintentionally pull on one side of the bar (back hand in my case).
Suggestions to fix:
1. Consciously stop doing it and correct if necessary mid trick.
2. Take the hand off the bar during the trick
3. Move both hands together on the opposite side of the bar.