A phrase from the PC Truck-racing game, Big Rigs.
In Big Rigs, it is impossible to lose, and every time you win a screen comes up with the words "You're Winner!" on it.
It can be used to denote a situation where the person addressed with the phrase is getting screwed over somewhat. It can also be used when the person being addressed did something grand, or got something grand.
People at Life, The Universe, and Everything use "You're Winner!" sarcastically.
"My friend just died..."