Apparently, the most recent system update to the Switch has brought with it a weird glitch where the console takes screenshots on its own randomly, without the user pressing the screenshot button.
Numerous Switch owners have reported the issue on the official Nintendo support forums and on social media. The bug causes the console to take screenshots on its own, even sometimes waking the console from sleep to grab a screenshot.
Some users have reported that a hard reset of the console works (hold the power button for 10 seconds to turn off, then turn back on).
Update: “2017-08-26T12:00:00Z”