Radak on Genesis
ותלך, after she had thrown Ishmael under one of the small trees, she went away a distance and sat down.
Haamek Davar on Genesis
And she went to sit across from him: A second time, to indicate that she distanced herself more than a bowshot, since she wanted to cry out loud. But she did not want the child to hear her crying; as this is hard for a sick person - whether he will recover or whether he will die. Hence she distanced herself further and she raised her voice and cried.