The neutrals seem angry that West Ham's first goal was chalked off.
I have seen the replays and don't think you can tell the moment the header is made, it is a nod down rather than a header you jump for.
They are assuming the moment the ball goes toward the head that is the moment of contact, don't think that is right as the ball arcs downwards obscuring the head and then the header is made. Microseconds yet that could explain the offside yet guess we will never know.