Albert Barnes Commentary 1 Samuel 29:3

"Then said the princes of the Philistines, What [do] these Hebrews [here]? And Achish said unto the princes of the Philistines, Is not this David, the servant of Saul the king of Israel, who hath been with me these days, or [rather] these years, and I have found no fault in him since he fell away [unto me] unto this day?" — 1 Samuel 29:3 (ASV)

He fell to me - The regular word for deserting and going over to the other side. See Jeremiah 37:13; Jeremiah 38:19.