When someone reports something as Copyright infringement, they typically don't have to offer proof to sites like Stage6/Youtube. They just take it down. Chances are it was a griefer who caused the video to go down by claiming that they owned it. I've had similar happen with a GW video of mine on some other video site...

Blizz has been very open that Machinima is not an abuse of their copyright.

Here is their Fair Use guide.
What this means for the artist

As a community machinima artist, you are permitted to create machinima productions, and to distribute them freely on your website, or on other websites where viewers can freely view your Production.
Anyway, just a guess. Malicious folks will lie just to get your movies taken off popular video sites (i.e. YOUTUBE).