How is this behaviour called?
The general term for the behaviour that you are describing is BM (short for "bad manners"). It is generally considered bad sportmanship, as already mentioned, but if you watch competitive Hearthstone you sometimes see players BM each other. The reason that this is OK, is because they know each other (maybe are even close friends) and take it as a joke. It is also quite entertaining for the viewers.
What are the reasons for it?
On the ladder players may do it to show dominance, often accompanied by several Sorry-Emotes, but the most common reason is finishing daily quests.
Maybe the xp-gain mentioned in the other answer plays a role too, but from my experience the it is all about finishing quests, like "Destroy 40 minions" or "Play 20 minions that cost 5 or most".
How to handle?
Since Blizzard made the change that conceding the game is not part of the action-queue anymore, you can just concede to skip all the BM your opponent may throw at you.
Before this change you would have to watch all your opponents actions if he queued them up before you pressed concede. Since they fixed this it is instant and saves you all that stuff. So best advice is to concede if you opponent has obvious lethal on the board.
However, there are situations where you dont know if you are dead and your opponent does a lot of other stuff before he kills you with his topdecked fireball. In this case you cant do anything but stay calm and queue up for the next game :)