Mourinho instructed his players to disrupt every Chelsea attack with fouls especially on Hazard and Willian, hence he put Phil Jones and Darmian to closely man mark the two. Conte on the sidelines saw this and was asking for a yellow card for every foul. Mourinho became frustrated, hence his reaction, but Conte did not buy any of this, you can see him in the video shouting "play the ball" at Mourinho.
Just before Herrera got sent off, Phil Jones fouled Hazard and Michael Oliver the ref called United's captain, Smalling, and told him "No more fouls, enough is enough, the next player to foul Hazard gets a yellow card." Seconds after that, Herrera fouls Hazard and he's sent off.
I'm having a laugh, stop watching all the Youtube videos. You could hear the Burnley fans at the week-end singing "Is this a library?" at Anfield and it was only a handful of them.
In the FA Cup 20% of the stadium capacity is given to the away fans. There was 9000 United fans last night, the ones that go with the team for every away match. The same atmosphere you'd have got had Chelsea brought the away fans at Old Trafford, or Arsenal their away fans at Anfield. You can't judge a fan base in England by their home matches as there is a lot of tourists these days and companies sharing tickets to their employees. Away match days are the real deal and I'm sure you'd change your mind if you'd go to a match with the Chelsea fans. Or you could tune in to the next away game and hear them singing throught the whole match.