I am going to take a wild guess here
And this is purely a guess, but looking into my crystal ball it appears that the motor was just ready to go. You could have had worn brushes in the P/N nozzle and when you replaced the belt the added stress was just that little bit more that pushed it over the edge.
As I said, this is just a guess; but I too am at a loss for why it would suddently go. Another possibility, again just a guess, was when you were doing preventative the movement wiggled some of the already loosened wires in the motor and then the rest is history.
Sorry I could only guess, just can't come up with anything else. If you decice to autopsy please let us know the results.
