I dont know how I exactly I triggered it, but in my last game I somehow managed to get more than 7 minions on the board while trying to magnetize my rover (while my turn was ending)
My board lineup was:
Windury tier 2 mech x2
Leeroy
Rover
Ancestral Automaton
Grease bot
Charging Czarina
Arcanotron
Clunker
I honestly have no Idea how it happend, but if it helps I was playing in the Phone version of the game.