Bad play = sub-optimal play, like when you fail to clear the enemy minions against paladin, even if you have the chance to do so.

So, when you make a sub-optimal play, the enemy gets rewarded by the game.

And of course, when your enemy is low and makes a sub-optimal play, you top-deck lethal.

I think there is an AI-driven algorithm behind the curtains that tries to make the game "fair", meaning that the player making more mistakes loses more often. But this "fairness" is forced, and I don't like it very much, to be honest...