Gotta put on the spare, but what the heck happened?

I'm still going to stand by G-Shock, they've served me very well. GD120CM-5. What fault is this in which the backlight activates, seemingly spontaneously, and doesn't turn off while the LCD goes blank? It happened yesterday morning during an emergency call and I didn't notice until after.

Also, forgot what model but I think I got it for Christmas in 2016 and kept it as a spare while replacing straps every few years.