It might be the slow accumulation. 5 or 6 obvious yellow cards ignored, two yellows for one of ours on the 'letter of the law'.
Remember Rodri escaping 3 or 4 times at the Emirates and eventually getting booked for taking his shirt off after scoring the injury time winner. And the travesty of ignoring that second yellow for Kovacic this season.
That's how a ref influences a game. Not with the big decisions that VAR can overturn, but by stealth. At Anfield, Rodri was booked for a poor challenge. On three subsequent occasions he had to avoid dragging someone down and preventing a break because he was on a yellow. Against us he can do it all day. Slowly, it makes a huge difference in a game.