It was very obvious that Mike Dean did not want to go red here, and another example of no one knowing what a clear and obvious error is. He was looking straight at it real time and immediately blew his whistle to stop the corner being taken so there is no suggestion that he didn't see the incident.
The problem here is with Lee Mason, how on earth has he watched that multiple times and deemed it VC? I wouldn't be at all surprised if PGMOL have told the referees not to go against the advice of VAR if they are sent to watch it on the screen. This is a mess, and is likely to be another situation where VAR has intervened and the decision will be overturned on appeal.
I really feel sorry for Mike Dean here, he is going to get the blame but it was obvious from his body language and the amount of time he took looking at the screen that he didn't want to go red. Yet again I say it is not the technology that is at fault, rather the guidance and directives of how to use it.