From the film, we also never see how ennis and cassie actually empassioned themselves. but, from the type of gal Cassie appeared to be and the extent to which see seemed to pursue ennis, he probably was pretty aggressive with her too.
So, I think ennis had it in him and exhibited strongly passionate behavior on occasion with certain people. Alma knew he was a bucking bronco, so to speak, and her shock was seeing it displaced to someone else.