NigelOnDraft,
You are absolutely right in what you posted above.
I spoke to a UA flight attendant in Tokyo a few days ago. She told me the evacuation was initiated after the tower reported what they believed to be a fire in the tires on one side of the aircraft. Pretty hard to ignore that, it seems.
By now all the "interested parties" should be well into the investigation, and with the benefit of hindsight and having ALL the information, should come up with their report before too long.