Albert Barnes Commentary 2 Samuel 20:6

"And David said to Abishai, Now will Sheba the son of Bichri do us more harm than did Absalom: take thou thy lord`s servants, and pursue after him, lest he get him fortified cities, and escape out of our sight." — 2 Samuel 20:6 (ASV)

To Abishai — Probably, because the king was on bad terms with Joab and wished to remove him from his position as captain of the army, he gave his orders to Abishai. He then weakly connived at Joab carrying them out, which was inevitable.