Jamieson, Fausset and Brown on 2 Samuel 15:7-9

7-9. after forty years--It is generally admitted that an error has here crept into the text, and that instead of "forty," we should read with the Syriac and Arabic versions, and Josephus, "four years"--that is, after Absalom's return to Jerusalem, and his beginning to practice the base arts of gaining popularity.

my vow, which I have vowed unto the Lord--during his exile in Geshur. The purport of it was, that whenever God's providence should pave the way for his re-establishment in Jerusalem, he would offer a sacrifice of thanksgiving. Hebron was the spot selected for the performance of this vow, ostensibly as being his native place (2Sa 3:3), and a famous high place, where sacrifices were frequently offered before the temple was built; but really as being in many respects the most suitable for the commencement of his rebellious enterprise. David, who always encouraged piety and desired to see religious engagements punctually performed, gave his consent and his blessing.

2Sa 15:10-12. He Forms a Conspiracy.