Another possibility is that the return beam from the mirror precisely hits the output facet of the laser diode. While this is a very small area, it only needs to happen for an instant. The result is an extended cavity which suddenly has a much lower loss due to the higher reflectance of the external mirror compared to the cleaved facet. The result is a virtually instantaneous increase in intracavity power and if the laser was running close to the COD (Catastrophic Optical Damage) limit, poof goes the laser diode. This would be more likely with a constant current driver but even in constant power mode, the increase in intracavity power would take place in less than 1 nanosecond - much less than the response time of the feedback circuit.