Originally Posted by
Rod Sheridan
A chip was probably hot enough to ignite in the airflow of the vacuum, exactly as happens in dust collector fires in some cases.
That was my initial idea / post.
I have changed my opinion. The OP's workshop has a subterranean deposit of rare Unobtainium, that creates a lens effect of cosmic rays. The Festool vac, just by happenstance, was sitting right in the direct line of the focused rays.
So - just don't let it sit there in the future. And - don't forget to wear the tin foil hat.
When I started woodworking, I didn't know squat. I have progressed in 30 years - now I do know squat.