battery drain overnight

Hi, it happened only once, but I really hope it will never happen before a race. I charged my garmin in the evening to 100% and disconnected it from charger than put it no my arm. In the morning I saw only 10%! It never happened before. I manage to charge it using a powerbank while biking to 55% and continued the ride. Now it is still 34% so it is discharging in right speed. But why it happened from 100% to 10% in just about 7 hours?