I had a U shaped quick link fail like that after years. Theoretically maybe the slot was never fully on the pin. Or the chain/chainring got bumped loading the bike in a car or something. I hear they wear out too/aren't very reusable.

Threw out all my U shaped quick links after that, which have a plate you bend to get them on. Nowadays I only use the symmetric quick links which are shaped like two letter L pieces. I feel the manufacturer can make these much stronger.