Grief
Elphaba's intention had been to fly as far away from the Emerald City as she could before she had no choice but to stop due to exhaustion. It was hardly a plan, but it would do for now. She knew she'd eventually have to land and deal with the consequences of her rash actions, but she hoped to put that off as long as possible.
But the further she was from the Emerald City, the further she was from Glinda, and everything she'd ever known.
It wasn't fatigue that soon forced Elphaba to submit to gravity, it was her tears.