Nasty. Glad it didn't come flying out completely. Time for a new collet. When you think of how little difference in inside diameter of the collet closed vs open--a couple of thousandths--it's amazing they hold up as long as they do.
A better description of what you were doing and what happened is needed. 1st thing I would point out is that no more than 1/16th should be removed in any pass and you need to be aware of the proper direction when routing. If you felt a sudden pull and the router got away from you then I'd say you started a climb cut which is the opposite of the direction you want to go and can pull the router away quickly. But a far better description is needed of what actually happened before anyone can give an accurate guess. A picture of the wood wood may yield a clue or 2 as well.
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