So is this being driven by people working on the VP8 codec or the WebM project? Or is MPEG LA just trying to show that VP8 really is just a patent encumbered at h.264(and protect/add to their revenue stream?)
Since google's adoption relies heavily on their claim that WebM isn't patent encumbered, it's a pretty safe bet that this is being done by the h.264 patent holders for competitive reasons.
Given that this is a press release from MPEG-LA hosted on its own website stating that MPEG-LA is asking for information about possible WebM infringements, I assume it's being driven by MPEG-LA.