This one is really tough, no doubt about that
Well, after yougave the last hint, I will give it another try. Taking in concern all of the above, maybe the motiv lies in reason that Heer generals were outraged by the fact that SS-Einsatztruppen couldn't care less about regular Wehrmacht chain of command. In fact, these special troops answered only to Heidrich and Himmler and I think that more than few high German officers considered them criminals with license to kill anyone. And this is something that Wehrmacht officers couldn't approve.
Hope I got closer now