Of Manasseh also there fell away some to David, when he came with the Philistines against Saul to battle: but they didn't help them; for the lords of the Philistines sent him away after consultation, saying, He will fall away to his master Saul to the jeopardy of our heads.
"They helped David against the band of rovers"  (1 Chronicles 12:21).  This band of rovers was that of the Amalekites who had plundered Ziklag during David's trip with the Philistines to mount Gilboa.  (See my commentary on  1 Samuel 30 for a full discussion of that Amalekite raid on Ziklag and David's victory over them afterward.)