From the photographic evidence in post No 1 alone, let us now try to see how many speculative causes we can postulate, in true PPRuNe tradition, before an obviously authoritative post emerges with the real reason.
They'll fall into two categories, I imagine; in the first it will be the aircraft that moved forwards while the tug didn't, and in the second the opposite. But why? There are so many possibilities!
Over to our usual experts.