As discussed in this thread:
https://discussions.apple.com/thread/...
After the El Capitan 10.11.4 update my iMac (which had run for 2 months without a restart) suddenly began to randomly freeze and then restart itself, very intermittently. Sometimes days between restarts, sometimes only minutes.
Now running 10.11.6 it's no better, and about to go back to the Genius Bar for another round of tests and **HOPEFULLY** a logic board swap, which seems to fix the issue for everyone else on the thread above.