Our Bunn makers, (airpot brewers model CTWF), will occasionally, like once a month, just starting blowing out steam and water without warning. It will definitely freak you out the first few times it happens. It's really not big deal, just get something in place quickly to catch the water. I never really knew why they do it but it seemed like the thermostat might have just gotten stuck and overheated the water until the over-pressure release valve let loose. We always just hit the button to run a batch of water and it always goes back to normal operation after that.
The Fetcos and Bunns I'm familiar with don't have an overpressure valve like there is on an espresso machine, just an overflow tube. On most, this tube runs down to a little port in the brew basket area.
What is your thermostat set to? If it is near the boil, small fluctuations may cause it to randomly boil. Check the temp of the water inside the tank with a good accurate thermometer and drop your set temp just a bit if it is too high.
On the Bunn CWTs (thermal pourovers) that have been plumbed in, water overflowing can be due to water pressure being too high. These ship with pressure reducers on the back, but sometimes they get lost when sold used.
There could be a number of things that cause your brewer to overheat or overflow. Time to call a tech.