Recently both belts on my Prusa MK3S+ became loose. Quite suddenly, too. One print was okay, the next nozzle wobbled in the Y and bed in the X causing layer shifting. More precisely, each movement was shifted pretty randomly.
I checked belt tension and there was basically none. It was good a month ago, and it must've been good enough just one print before. Of course I will tension them again, but I would be glad to know what are the possible causes of belts losing tension suddenly, to check them during the maintenance.