Albert Barnes Commentary 2 Samuel 19:26

"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 with the king; because thy servant is lame." — 2 Samuel 19:26 (ASV)

It appears that when Mephibosheth ordered Ziba to saddle the donkeys and ride with him to join David, Ziba left him, pretending to obey. Instead, he loaded the donkeys with provisions and went off alone with them, making it impossible for Mephibosheth to follow.