Power cubes failing at an increasing rate

We have two Sensor systems in our venues. One is almost 20 years old the other about 12 years old. We are experiencing power cube failures at in increasing rate. I've replaced 4 cubes (out of 136 dimmers) in the younger system and 10 to 12 cubes (out of 192 dimmers) in the older system. The symptom is typically a failed on condition. All of these failures are within the last three years, no problems of this sort before that. 

If it were just the older dimmers I would suspect it is just end of life of that component, though at $100 plus per cube it isn't "just." However the fact that the younger cubes are failing also confuses me.

Does anyone recognize this pattern? Could we be using the dimmers in a way that causes this failure? We do use a lot of dimmer doubling.