Only tidbit I can find on what triggers hyperphagia is the shorter and cooler days. Once that type of weather/ light begins, they'll begin the hyperphagia stage of hibernation. Don't know if that helps.
Whatever they say it is, it seems to be more speculation. So many unknowns it seems with bears. The article I found had most its information from the mid 80’s and I know they have some new information that wasn’t available then.
One bear my clients shot in six week in June ate two adult moose and 42 calves as well as beavers, squirrels, eggs, grass and even a swan.