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Scenario 1: It's clear that I'm going to lose a game. On my last turn, I saw that my opponent had enough on the board to kill me on his next turn. I looked through my cards and found that I couldn't stop it. I end my turn and all my opponent has to do is have a single minion already on the board attack my hero and that ends the game.
Some people will do the polite thing: kill me to promptly end the game so that we can both get the reward for playing and move on. But a large fraction won't. Some will sit there and think for a while, play three cards, buff minions, heal his own hero, kill my last minion (which isn't a taunt), and finally get around to killing my hero after using the bulk of the time allowed for his turn.
Why do people do this?
To be clear, I'm not talking about situations where my opponent needs to play a few cards, clear my taunt minions, buff his own minions, or whatever to have a complicated last turn to win the game. I can understand why it takes a while to think about that. But when all it takes is this minion attacks my hero with no secrets on the board and the game is 100% guaranteed to end with no further actions? I don't understand taking 40 seconds for a turn to do 5 other moves before ending it.
Scenario 2: It's clear that I'm going to win a game. I have it set up so that I have enough on the board to end the game on my next turn unless my opponent can do something to defend against it (kill minions, drop a taunt, self-heal, etc.). My opponent sees this, looks through his hand, and realizes that he's lost.
Some people will do the polite thing: end his turn, let me kill him, and we both get the reward for playing and move on. But an awful lot of people will decide to concede the match immediately instead. Letting the match last another 5 seconds would increase the experience rewards for both of us.
This one, I'm a little more sympathetic to. Some people may not care about experience; from the number of non-golden basic cards they're using, it's pretty obvious that they haven't hit the experience cap yet. But if you don't care about golden cards, so be it. And someone might reasonably worry that scenario #1 will play out, as seems to happen a lot. But I still think it's annoying.