Albert Barnes Commentary 2 Samuel 6:1

"And David again gathered together all the chosen men of Israel, thirty thousand." — 2 Samuel 6:1 (ASV)

Again - This refers to the full phrase, and David again gathered...; that is, after the previous gathering for either his election to the kingdom (2 Samuel 5:1–3) or the Philistine war (2 Samuel 5:17–25), he assembled them again for the peaceful purpose of bringing the ark to Mount Zion. The whole narrative indicates the progressive consolidation of David’s power and the settlement of his monarchy on strong foundations.