Albert Barnes Commentary 2 Samuel 20:8

"When they were at the great stone which is in Gibeon, Amasa came to meet them. And Joab was girded with his apparel of war that he had put on, and thereon was a girdle with a sword fastened upon his loins in the sheath thereof; and as he went forth it fell out." — 2 Samuel 20:8 (ASV)

Amasa went before them. Rather, “advanced to meet them.” Amasa was no doubt returning to Jerusalem according to his orders (2 Samuel 20:4) and was probably very surprised to meet the army on the march. Joab’s resolution was quickly taken.

And Joab’s garment... Translate as: “And Joab was girded with his military garment as his clothing, and upon it” (that is, the military garment or him) “the girdle of a sword fastened on his loins in its sheath, and as he went forth” (to meet Amasa) “it fell” out of the sheath. What appears to have happened is that, by accident or design, Joab’s sword fell from its scabbard to the ground as he went to meet Amasa. He then picked it up with his left hand, leaving his right hand free for the customary greeting (2 Samuel 20:9). This awakened no suspicion in Amasa’s mind. Compare the case of Ehud (Judges 3:21).