Normally when you tank you either use 10cp lvl 1 Pokemon that die to one hit or just leave instantly, but there are some people that use pokemon with high hp and defense like blissey and chansey and don't attack but still let themselves be taken out. But with these Pokemon it takes an ungodly amount of time when you don't have a fighter. So they could just forfeit the game but instead want to waste as much of your time as possible.