This happened to me just yesterday with the original SLA battery at around 70% charge. It may have been earlier in the day but I definitely recall it happening repeatedly from 70%-50% charge. I have used it 5 times (twice at end of last spring). No idea why...but it was similar to bumping my transducer while jigging aggressively.
I also noted that after using it for 3 hours and then putting it on the charger for the evening, it showed only 88% battery left when I turned it on the next morning. I believe it had previously recorded at or above 95% for the 4 previous outings after a full charge.