Good to see the system working and the red overturned. Important. Need MacAllister, he's playing well.
You still have to wonder about the ref, and the cabbages in the VAR room, on the day - but anyway, good news today.
Referee's make mistakes...always have and always will. In real time he see's the coming together, both players going for the ball. He perceives that Mac is fractionally slower to the ball than the other lad, misses the ball and catches him. The red card comes from the reaction of the Bournemouth player, perhaps he was hurt who knows.... but his exaggerated reaction was enough to convince the ref it was a red card. Cue VAR: VAR are instructed to not re-referee the game, they only alert the ref to something he can't have seen, usually something that can be measured, like offside or a foul inside the box. They don't disagree with a referee's subjective judgement. The panel that adjudicates the appeal obviously came to the conclusion it was a genuine accident.
VAR operators are also subject to information cascades, were their decisions are influenced by the preceding opinion. This used to happen when judging Olympic boxing, so to prevent it, the IOC introduced a system where each judge presses a button (a point scored) but the other judge(s) don't know what that button press means.