Some from Manasseh defected to David when he went with the Philistines to fight against Saul. (They did not help the Philistines because the Philistine rulers consulted and sent David away, saying, “It will cost us our heads if he defects to his master Saul.”)
We have here an account of those that appeared and acted as David's friends, upon the death of Saul, to bring about the revolution. All the forces he had, while he was persecuted, was but 600 men, who served for his guards; but, when the time had come that he must begin to act offensively, Providence brought in more to his assistance.
Commenting on 1 Chronicles 12:1-22
For at that time, day by day, there came to David to help him,.... Particularly after the defeat of Saul by the Philistines, unto the time that David came to Hebron: until it was a great host, like the host of God; the heavenly host, the angels; so the Targum.
there fell some of Manasseh--The period of their accession is fixed as the time when David came with the Philistines against Saul to battle. but they helped them not--(See on Sa1 29:4).