Burst disks flex every time the gun fills and empties. Over time this can fatigue the metal and the disk will rupture, even though the burst pressure was not achieved. I saw this all the time many moons ago when I worked in a SCUBA shop and was filling tanks. No matter how often it happened, you still jumped every time one let go.
Alternatively, if this is happening too often (in your mind), there might also be maybe a machining blemish on the seat or on the hollow bolt which holds the disk in place. If a burr or something is damaging the burst disk just a little bit when it is being installed, this will also lead to a shorter service life.
Or, since all burst disks you seem to be able to get these days come from China, it might just be a disk quality issue.
Just don't get to the point where you are tired of replacing them, and move to some with a higher rated pressure - DANGEROUS!