Albert Barnes Commentary 2 Samuel 15:24

"And, lo, Zadok also [came], and all the Levites with him, bearing the ark of the covenant of God; and they set down the ark of God; and Abiathar went up, until all the people had done passing out of the city." — 2 Samuel 15:24 (ASV)

Abiathar went up - that is, he continued to ascend the Mount of Olives. Abiathar was high priest (1 Kings 2:35). Perhaps Zadok is the one addressed by David in 2 Samuel 15:25 as the chief of those who were actually bearing the ark.