Dirty and or loose connections is the most common reason. Most of us keep a 10 mm wrench with us and check the terminals before we ride. There is 50 g's of vibration at the battery box when accelerating. Things get loose.
That being said, that is another reason we run the Deka battery, instead of a cheaper brand. That battery is designed to handle the vibration. You start shaking the inside of your battery apart, you will have all sorts of electrical problems.
Make sure your connections are clean otherwise it’s like being a resister and gets super hot from the amount of current passing through. Clean and tight is important. Does the bike start strong? A lower battery voltage will pass a higher current to start the bike too.