Odd that you should post this - it has happened to me in the past, on various of my machines, and I never found the cause.
Today it began happening, for the first time, on my iMac (which I've had for two years now, without such an incident). Very odd.
I think, in the case of today, it may be related to my iPhone, which has a wonky charge / data port, and with which I'm having a bunch of problems. Even Siri (on my Mac) began suddenly asking me repeatedly what she can do for me, unprompted, and to the point where I had to shut it off in system preferences.
Maybe a solar flare / coronal mass ejection messing up with the electromagnetic fields on earth today? All I know, it's been one of those very strange days, computer-wise....