This is a common symptom of intermittent contact. That comes in one of two ways:
- The flash isn't fully tight in the camera's hot shoe. Even a little bit of slop in the mount can cause the contacts to not stay tight against one another, and certain combinations of contact can fire the flash off for some reason (I don't understand why we have a design that would allow this, but it's a reality we have to live with). The problem is that the hot shoe and flash foot are metal, and metal expands and contracts with temperature, so some tolerance has to be built into the connection. Unfortunately, it doesn't take much slop to trigger the problem. While I don't recommend it, I've seen people fix this problem by slightly bending the wings on the camera's hot shoe to keep the flash a little tighter to the camera. But bending the wings isn't a great solution, as it's too easy to go too far.
- The foot of the flash has internal connection issues. When flash units get dropped or banged against while in the camera hot shoe, the wire connections in the foot can get damaged. All it takes is for there to be an intermittent connection and you can see inadvertent flash triggering. The solution here is to get the flash repaired.