I think there's a mixture of favouritism and incompetence at play, but probably not outright corruption. Clattenburg, for example, has openly admitted that he heavily favoured United when refereeing them under Fergie. Pretty sure they were mates too, which seems like a conflict of interest.
Referees aren't helped by the rules of the game being incredibly unfit for purpose and vague either.
As for a conspiracy theory, McTominay and El Shaarawy are still in their early 20s. There's no way McTominay is 27. I'm not buying it.