Bicycle components fail for many reasons including, but not limited to:
• Poor maintenance (chain breaks because it was never lubed)
• Impact/crashes (driving into your garage with bicycle on your roof rack)
• Design defects (hollow cranks that are too hollow)
• Manufacturing defects (improper frame welding)
For some chilling reading, go to the Consumer Product Safety Commission website and type in "bicycle" or the name of any major bicycle manufacturer or bicycle component manufacturer. You could spend many hours reading about hundreds of recalled bicycles and bicycle components. Try searching for the name of the manufacturer of your mountain bike fork and see what comes up.
Although I am using fictitious names here, these are real life examples. "ABC Corporation" voluntarily recalls 220,000 front suspension forks because compression rods can break, causing rider to lose control, fall and suffer serious injury. "XYZ Corporation" voluntarily recalls 17,500 BMX and mountain bike forks because the steerer tube can fail, causing loss of control and injury.
Over the years, I have handled many bike defect cases, including claims for defective forks, pedals, seatposts and frames. In addition, I have personally witnessed crashes involving defective handlebars, forks and wheels. It happens when you least expect it and it's never a pretty sight.
Mountain bike forks are a fertile source of defects. After spending two full days deposing the R&D personnel from a major fork manufacturer, I learned the specific reasons why the particular fork failed and also that there was grossly inadequate product testing. The manufacturer was in such a rush to get the latest, greatest and lightest fork on the market that it was doing minimal product testing and essentially using the consumers as a guinea pig. Reported failures and defects were used to redesign the newest version due out 9-12 months later.