I've experienced the same thing recently on my 2012 Mini and I'm pretty sure that's never happened to me until recently. It started happening, maybe once a day, for several days straight but it hasn't happened for several days now. There was also a thread on the El Capitan forum (I think) about the same problem - but in that case, the very specific things they were seeing in the logs was not was not what I was seeing.
For me, the problem was likely Flash in Firefox. I know this because I would go to one of my other computers, log in to my frozen computer, kill the Flash plugin in Firefox and the computer would unfreeze. (There was a recent Firefox update which might have cleared up the problem.)
If you only use Safari and you have Flash, I suggest disabling it and seeing what happens. In Safari, go to Preferences, select the "Security" button, press "Plug-in Settings", and uncheck Adobe Flash.
Even though you use Safari and my problem is with Firefox, I have both opened at the same time. Until I disabled Flash in Safari, I was getting kernel panics (WebKit was the process running when it happened according to the logs). So an improvement at least. I just received notification of yet another Flash update a few hours ago - maybe they fixed it.
I don't know if El Capitan 10.11.4 helps or not, I have not yet updated from 10.11.3. I'll need to look for the other thread - I saw it and when I looked later, it wasn't there or I wasn't looking properly.