Player handbook section on Opportunity Attacks says (pg 195):
You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy.
From this I'd say letting go is being moved without using the sorcerer's movement, they don't need to turn their back on the snake or focus on moving themselves. The specific example given is a person falling past an enemy (into and out in one fall, as I read it) so granted they would probably be moving faster but I'd still say that no, falling doesn't cause an opportunity attack.