Actually, you can't argue with the donkey owners logic. If I were to rear end someone, it would be my fault. Sounds like the motorcyclist was inattentive.
And, what we all want to know-- was speed a factor?
Seriously, I can't imagine a motorcycle crashing into a donkey would be good for the donkey. I'm surprised the cyclist had serious injuries, but none are mentioned for the donkey (aka, the "other vehicle").
Seems to me, one of my vocabulary words about 50 years ago was "vehicle" and I never got to the part about the donkeys.