Absalom starts his campaign to undermine his father by parking his chariots outside the city gates and intercepting anyone coming to David to solve disputes and telling them he's the only magistrate available. He gradually gets the entire nation behind him with his sound judgements.
After 40 years of this, Absalom tells his father he vowed to return to Hebron at some point, and he thinks now's the time. While he's in transit, he sends his men around to say that when the trumpets sound, Absalom is king.
Finally, one of David's men hears about it and warns him. He flees, leaving his 10 concubines behind, but taking his 600 most loyal followers. We get a couple of scenes of David having heartbreaking conversations with his followers, sending them back and telling the bearers of the Ark of the Covenant to go back to the city lest god think him disloyal.
At one point he goes up a mountain to pray, and one of his followers begs him to go back to the city and ask to be Absalom's servant.