This is a great situation about co-operation with the offside decisions, although the attackers are impeding the goalkeeper´s actions and offside should be called.
1) The referee has raised his hand to indicate that it is an indirect free-kick. This means that he has to focus on whether there is a touch for the goal to be allowed. The assistant at the same time is focused on the second last defender and very likely does not see if someone touches the ball - so he needs help from the referee.
2) The attacker touches the ball with his head then a new offside situation is created and the attackers that where onside, when the kick was taken, are now in an offside position. The assistant needs to know about the touch. Therefore referees are reminded that when using head-sets they should pass such information immediately.
3) The referees have to decide whether the attackers are impeding the actions/movements of the goalkeeper. Here they are running and jumping towards the ball. This has impact on the goalkeepers ability to play the ball as he does not have the opportunity to jump for the ball (he would be jumping into the feet and raised knee of the in-running attackers).