The FA found that United failed to "ensure their players conducted themselves in an orderly fashion" by surrounding referee Craig Pawson following a controversial decision to initially award Liverpool a goal.
The incident occurred after Roberto Firmino scored following a challenge by Virgil van Dijk on David de Gea: the United goalkeeper dropped the ball under pressure, and United felt that Pawson should have awarded them a free-kick.
Following a VAR review, a foul was awarded - but United players had reacted furiously to Pawson's initial decision not to award a foul, with De Gea booked for his protests.
VAR drama! 😳
Roberto Firmino thought he had doubled Liverpool's lead, but it's ruled out after a foul on David de Gea in the build-up.
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The decision provoked much debate, with former United captain Gary Neville saying at the time: "I don't think that's a foul. I think he just dropped it. It's soft."
The forceful nature of United's response - particularly De Gea's - may partially be down to the goalkeeper feeling aggrieved at not feeling protected by referees during aerial challenges this season, having seen a similar decision go against him as Everton drew 1-1 at Old Trafford on 15 December.