John Gill Commentary 1 Chronicles 12:19

"Of Manasseh also there fell away some to David, when he came with the Philistines against Saul to battle: but they helped them not; for the lords of the Philistines upon advisement sent him away, saying, He will fall away to his master Saul to the jeopardy of our heads." — 1 Chronicles 12:19 (ASV)

And there fell some of Manasseh to David
Of the tribe of Manasseh; they took his part, and on his side, and joined him:

when he came with the Philistines against Saul to battle ;
which was a strong proof of their cordial attachment to him:

but they helped them not :
the Philistines; neither David nor his men, nor the Manassites that joined them:

for the lords of the Philistines, upon advisement ;
counsel taken among themselves: sent him away; that is, David and his men:

saying, he will fall to his master Saul, to the jeopardy of our heads ;
meaning, that he would go off with his troops to Saul, and betray them into his hands, and with their heads make his peace with him, see (1 Samuel 29:4) .