Loading an assembly by clamping on a thin walled hollow space while trying to make a peripheral cut produces a whole lot of poorly supported force. Seems I've learned this lesson somewhere in the past myself. Very glad you survived the ordeal with nothing worse than a lost burner.
A file may have worked better at removing material in that set-up, as it is less prone to dig in and grab, but a slow gentle approach would still have been needed.
Very nice burner by the way, I'd say it's about perfect for that engine ..... well done!!!