All locks have failure modes, some moreso than others. Liner locks are known to fail for numerous reasons. The most common by far is failure to engage fully initially. This can occur for a number of reasons and unless you eyeball it before each use, you will not notice it. They can also be inadvertantly released, the lock can literally bend, slip due to contamination, wear, etc. I've had numerous Buck CrossLocks that worked just fine, but they are certainly subject to the usual common failure modes. If I were to bet, I'd say it didn't engage fully originally.