How does a timing chain stretch and oil pump chain guide get damaged at the shop

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Oil pan gasket replaced at Dealer shop (extended warranty). After repair a test drive was completed. Car stalled. Started rough and driven back to shop. VANOS system failed, timing chain stretched, oil pump broken and the oil pump chain drive is damaged. Regular maintenance. No noise, no timing issues, solid performing BMW. What caused this to happen in the shop? Shop quoted $9200 and warranty only covers $5000. They lowered to $7900 after we posted a review and reported to BBB. How does this catastrophic damage happen when it was running perfect at drop off? The Service Department and Manager are giving us the run around and won't let us speak to the Director. Please help us understand!