What factors likely led some early americans to change from hunting to farming?

Farming is more reliable: it produces more regular source of energy, as you can store your crops for winter and suffer less if you can't find a prey. Additionally, you need less energy to farm, so you have more time: you don't need to spend all your time and energy chasing food.

This is the reason why people generally in history switched into farming, and those reasons are also valid for Americans.