SM's are burning

Recently I had four SM’s gone bad – these are SM’s that have been working for over a year. The SM will connect but they will drop. After inspecting the SM board I noticed that they seemed to have overheated. They have four heat dispensers that appear to be damaged.

I find it strange that suddenly four SM’s that have been working well for over a year suddenly ‘burn’. I should say that all of these customers had their SM connected to a UPS, also they are connected to different AP’s on different segments of our network (not that I think the AP has anything to do with it). All of them are board P9 and are running on 8.1 or higher.

I guess my questions is should I write this off to bad luck and strange coincidence or is this a know issue? I would include a picture for reference but I am not sure how to upload it.