Ok obviously the hot tub did not “know” the ex-wife was cheating on the husband.

The hot tub just wanted him for themselves. So they started acting petty to the ex-wife. It was only a coincidence that the ex-wife turned out to be cheating.

Then the ex-wife leaves. The hot tub is happy. Finally, just them and the man.

Then the man starts talking about some “Emma”. The hot tub sees the way he talks about her. But they aren’t sure which Emma it is. So they start acting up whenever an Emma comes around.

Pretty simply explanation if you ask me.