John Gill Commentary 2 Samuel 19:1

"And it was told Joab, Behold, the king weepeth and mourneth for Absalom." — 2 Samuel 19:1 (ASV)

And it was told Joab
When returned to Mahanaim, or on his way to it; perhaps by the messengers he sent; and this report he had before he saw the king:

behold, the king weeps and mourns for Absalom ;
for the death of him, instead of rejoicing at the victory obtained, and the deliverance from his enemies.