My initial thought watching the game live was WTF...
Just watched the goal again on YouTube, there was a Chelsea player close to the keeper when the corner was crossed in, can only imagine they may have thought the player was interfering with the keeper, but when Silva headed the ball the player was nowhere near the keeper.
VAR offside check for the 4th goal seemed to take an eternity, although we know the process they have to go through, would help the viewers who don't know the process if commentators weren't so negative all the time whilst the check is taking place.