Because it has zombies and Valve likes to put out hints beforehand for their upcoming games. And Valve has announced stuff at Spike before.

Cool... I just installed the first one on my laptop, and I have to say, the game is far more playable then I remembered, especially in co-op

Honestly speaking I am just glad they didn't release one this year. That is all I can say. 

If this is left 4 dead 3 i will personally slit Gabe newells throat

that is all

Yeah, but it's not like Valve is the only place that wants to announce something and it's not like Valve makes the only zombie game.

Personally, that really doesn't look like it fits in Left 4 Dead at all.

People assume it's Left 4 Dead 3 because it's written "1 of 3" in the bottom right corner.

That really doesn't make a lot of sense either.  Why would Left 4 Dead 3 be 1 of 3?  Left 4 Dead has never really been called a trilogy...

It's probably going to be the debut trailer for that Japanese Zombie game. I can't remember the name or who's making it but I read something about it a couple weeks ago. It's supposed to launch sometime in 2011 I think.

edit: I think it'd be NeverDead from Konami

that game already debuted and looked crappy to be honest.

I should also point out that the Normandy appears very briefly just before then