Albert Barnes Commentary 2 Samuel 20:5

"So Amasa went to call [the men of] Judah together; but he tarried longer than the set time which he had appointed him." — 2 Samuel 20:5 (ASV)

He delayed—The cause of Amasa’s delay is not stated. It may have been the unwillingness of the men of Judah to place themselves under his orders, or it may have been caused by a wavering or hesitation in his loyalty. This last possibility is evidently insinuated in 2 Samuel 20:11, and no doubt this was the pretext, whether grounded in fact or not, by which Joab justified the murder of Amasa before David.