Happened to me a few times. I don't think they are bots, as bots would have noticed I have lethal on board and cleared it.
Maybe they just gave up on the game because they know they already lost, but don't want to concede because quests and achievements and disconnection and whatnot, so they just braindead go face until they die, me personally I'll just pass my turns really quick but I guess everyone has their own thing.
I'll say that it's really frustrating that Blizzard doesn't award quest progress on a concede in the specific instance that *lethal is on board* for the opponent. I get it that you don't want players queue-conceding through the daily quests. But playing out a game and then having an opponent with more attack than you have health should be sufficient to bottom-right safely.
You do get credit when you concede.
You just don't when you concede on Turn 3 or 4
Just to be specific here, because it might help some people, you get credit when you concede turn 10 or when you are at or below 15 hp.
I only have Warlock, Priest and Warrior cards so i just built suicide decks for the firelands quest chain were you have to play shaman etc, lightning bolted my own face, gave opponent some bananas and conceded as soon as i hit 15 hp. Like i said it might help some people.