Place another MOV across the relay contacts, or a series snubber of 22R 0.5W and 0.22uF class X capacitor. Likely arcing of the contacts is doing this, with the very large toroid acting as an almost pure inductor.
I had the same with my variac until I did this, as it is also a toroidal transformer of around 2kVA. Turn on surge would trip the 30A mains breaker.