I heard a good few rumours. Here are two that i heard
1: The Drummer had broken his leg and therefore couldnt play
2: As dumb as this rumour may sound, Vital Remains thought that they were too "big" to play Wacken (It's a really stupid sounding rumour i know, and i wouldnt want that idea in my head, but i heard rumours like that cropping up)
Sorry about the rumours