The Premier League explain why Son received a red card

Although Andre Gomes seemed to get injured after the subsequent clash with Aurier, Son was sent off. His reaction to the Portugueses' injury was one of panic: he regretted it the moment he went to the ground to get the ball and could not repress his tears.
"The red card for Son was for putting the health and safety of a player in danger as a consecuence of his challenge," the Premier League explained a few minutes after the Everton-Tottenham game ended in a draw, although the result was the last thing on anybody's minds due to the horrific scenes involving Andre Gomes.
Andre Gomes was knocked down by the Korean and ended up seriously injured. You only had to look at the right ankle of the Portuguese player.
Not only Son, but also Aurier could not hide his sadness after the injury of a fellow footballer. "He is devastated and he is a good guy", Tottenham players stated about the striker after the game.