Jo was not strong enough to keep Dean in the air with her. So when he started to fall, he slipped out of her grip and she kept floating, unable to control which direction she was going.
"Dean!" she shrieked, for all the good it did.
It was better that he'd landed in a tree instead of falling all of that way to the ground, but still not good. Nor did it bode well for whenever her flying spell decided to shut off.