Looks to me like the aircraft had a steering problem on the taxiway and turned right onto grass - reason unknown.
The tug is after the event, being attached to nose leg probably to facilitate pushback onto taxiway.
Absolute rubbish!
Those who know Frankfurt ops will know that, after deplaning its passengers at the terminal, the Air China is towed to a remote parking on the south side. Therefore, a technical problem with the tug or, as already mentioned, that the tug driver became ill.