I have a 2015 top-spec iMac 27 running OS X 10.11.4 and have seen this a few times. I had several support calls to Apple and the last action was resetting NVRAM and SMC:
https://support.apple.com/en-us/HT204063
It still spontaneously rebooted a couple of times after that, roughly once per day, then it quit -- all by itself. When the reboots happened, I was not running anything on the machine at the time -- it was just sitting there idle.
I have seen a few other reports of this with some speculation maybe it's related to 10.11.4, but no proof of that.
I don't know how widespread this is but even if it's just a few cases Apple must know about it through their tracking system. I understand your frustration -- this should not be happening. My suggestion is diplomatically but firmly and persistently follow up with Apple. If escalated to the proper level, they have the ability to analyze your system logs and crash dump files to try and home in on the problem.