Ed is correct. As a result of these imbalanced rotations the struts - which are holding the engine in its position - brake and the engine starts to change its position in the airframe and moves foreward and backward ... a soft gliding becomes impossible.
Ben,
the E.III was only a replacement aircraft. He used the E.III because his E.IV was damaged in the aircombats earlier this day. I am not knowing the markings but maybe one of the modelers knows more.
The leftover of the airframe with the engine and the pilot crashed down together (he did not jump like others) but - as often happened in other cases before and after - the pilot came under the engine.
Long time ago somebody was displaying a photograph with Immelmann within the wreck here at the Forum. I lost my copy meanwhile. I have seen two other pictures of the crash site and one or two of the parts of the wreck (maybe prepared for the examination). I am not knowing the real numbers of photographs taken there. I was not there.