Adam Clarke on 2 Samuel 15:33

Then thou shalt be a burden unto me - It appears that Hushai was not a warrior, but was a wise, prudent, and discreet man, who could well serve David by gaining him intelligence of Absalom's conspiracy; and he directs him to form a strict confederacy with the priests Zadok and Abiathar, and to make use of their sons as couriers between Jerusalem and David's place of retreat.