What caused Vespasian's downfall?

Vespasian did not suffer any downfall in the usual meaning of the term. Yes he did eventually die, but he was nearly seventy and had secured the succession of his son Titus and the Flavian dynasty after him, no small feat considering the many emperors who tried and failed to establish dynasties. His last Words before expiring were "Væ, puto deus fio" which means "Oh, I think I am becoming a god!" Certainly not Words spoken by one who was suffering a downfall.