go back to the motor and look at the armature bars real close.
that glowing you saw was the residue or possibly a chunk of copper off one of the bars shorting against the next bar.
There is one critical step alot of guys forget to do or just don't know to do and that is to clean out the gaps if your doing alot of resurfacing and then following up with a polishing stone.
So go in and check between each bar and make sure there are no bridges of copper built up. You can take a tool they make for cleaning the gaps and truing up the edges of the bars to make a clean gap.