I had this happen last week too. Not sure why; it's the only time I've encountered it. I was just playing, and suddenly it got very loud and distorted... so I touched the main volume knob and it reset to the level I had previously been using.
I've had occasional other issues with values jumping though. Had to replace the mainboard on my synth because of it. That issue is quite a bit better now, but not completely absent. Every once in a while, I'll have a parameter jump to a new value without being touched... and occasionally when I turn the synth on, it fails to calibrate the touch strip, so it acts like I always have a finger on it at one end or the other. But a quick power cycle has always resolved the issue.
It seems there is a bit of electrical noise (perhaps from my unfiltered mains power) causing spurious readings when the main board checks the position of its input devices. The new board increased a resistor value to dampen the spikes, and it mostly worked, but it may perhaps not be dampened quite enough to eliminate the issue completely.
At some point, I should try a power conditioner to see if it it eliminates this sort of issue. It would probably also help with other things, like reducing my noise floor. But that's an expense for some other day in the future.