I wonder about your timing ...
Casting the heavier wind resistant poppers you probably have to wait a second longer for the line to straighten behind you compared to casting the smaller fly. With the larger popper you may also feel a more noticeable tug on the line when the line straightens. The lighter fly will get back quicker and also not signal as clearly it is time for the forward stroke. It may be that you catch the trees on an interrupted back cast, which fools you into believing your back cast always stays high, but on a normal one I wonder if the line is starting to fall before you begin the forward cast. Then to get the feel you are used to (from the heavier popper) you are driving the rod too hard at the beginning of the forward cast in order to get the heavier feel you are used to. If you refine your sense of touch things should improve, IMHO.