Mojave screen saver starts randomly

I have a new Mojave (10.14.1) installation on an iMac (Retina 5K, 27-inch, 2017). My screen saver is set to start after 5 minutes. I have a hot corner defined to start the screen saver.

I have noticed that periodically, the screen saver just starts up out of the blue. Typically this happens while I am scrolling with the magic mouse. I am definitely not hitting the hot corner.

Any suggestions for why this is happening and how to stop it?