I too suspect it may be more to do with Michael Oliver's role in a Real Madrid v Juventus game 2 or 3 years ago. He awarded a penalty and sent off the Juve GK near the end which put RM through. The general consensus was that both decisions were justified and Oliver was supported by Collina. He and his wife Lucy received all sorts of despicable threats subsequently (referred to in another thread).
As MO is VAR for the West Ham game on Monday, he could only fly out Tuesday which makes Sevilla-Dortmund the only option being a Wednesday game.
Taylor on the other hand could in theory do Barcelona - PSG on Tuesday.