What appears to have happened is, that when Mephibosheth ordered Ziba to saddle the donkeys and ride with him to join David, Ziba left him under pretence of obeying, but instead laded the donkeys with provisions, and went off alone with them, thus making it impossible for Mephibosheth to follow.
Related Commentaries of 2 Samuel 19:26
2 Samuel 19:26
26 And he answered, My lord, O king, my servant deceived me: for thy servant said, I will saddle me an ass, that I may ride thereon, and go to the king; because thy servant is lame.