Basically, warlocks will discard all of their cards and deck and then I automatically die, no health lost. I'll always have 30 health, and a board full of minions, it doesn't matter ... I even had like 12 armor on top of 30 health in one of the matches.
I think that's the condition, it seems like every time they're just using cards to get rid of cards as quickly as possible, and then as soon as they run out of cards my character just explodes.
Turn 8 or later, they play a cheap spell, then Dollmaster Dorian and then a second spell Plot Twist. This shuffles the few remaining cards back into their deck and redraws them. Due to Dollmaster Dorian being on the board, a 1/1 copy of each minion drawn is added to the board. These minions are Kael'thas Sunstrider (which makes every third spell free of mana cast) and Mecha'thun. The player then casts his third spell, which is Cataclysm. This empties their hand and destroys their board. Because Mecha'thun dies, you die (see its Deathrattle effect).
I suggest you install Hearthstone Deckt Tracker (or any variant of it) to keep track of cards your opponent play. If you had this App you would be able to see all cards they have played, including of course your cards.
Give it a try. Because then, if you loose a game like this you'll be able to view the whole deck your opponent used up to that point.