iPhone mystery lock

Friend's iPhone 11, running iOS 14.7.1 exhibited strange message when she tried to unlock it. Message read "iPhone locked. Try again in 5 minutes". During this time, the device was totally unresponsive. I've never seen this before in 15 plus years of iPhone ownership. Did a power cycle, but on power on the same message reappeared. After waiting out the 5 minutes, message disappeared and everything seemed normal. Anybody have an idea of what that is all about? Thanks.