The chili simmered OK from 6:00 AM to somewhere between 10:00 AM and 2:00 PM. That's somewhere between 4-8 hours of simmering. With the lid off, you cooked away a lot of liquid, reducing the quantity of food allowing the remaining food to get hotter and boil.
First, 2-3 hours is more than enough for chili. Second, the burner maintained a simmer for more that 4 hours. That's pretty good if you ask me.
A simmer burner does not absolve you of responsibility to keep an eye on the pot.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan