John Gill Commentary 2 Samuel 13:30

"And it came to pass, while they were in the way, that the tidings came to David, saying, Absalom hath slain all the king`s sons, and there is not one of them left." — 2 Samuel 13:30 (ASV)

And it came to pass while they were in the way
On their road homewards, before they got to Jerusalem:

that tidings came to David ;
perhaps brought by one who was at the entertainment, who upon seeing Amnon smitten, immediately rose up and fled with the news of it to David:

saying, Absalom has slain all the king's sons, and there is not one of
them left ;
which he might suppose was the design of Absalom, and was done before that time; however, so it was ordered in Providence, that David, that rejoiced at the news of the death of Uriah, might be terrified with the tidings of the death of all his sons; and for a while it was as if it was really so, which occasioned the following behaviour in him.