How did he hear about Tirhakah?
Malbim (Yeshayah 37:9): By chance, he heard from passersby that he left his land in order to fight him.
Why did he send to Chizkiyah again?
Rashi: He informed him that he is not returning to Ashur. After he fights Kush, he will return to Yerushalayim.
Malbim (Yeshayah 37:9): He knew that since he returns to Ashur, people will not rebel against Chizkiyah. He wanted that Chizkiyah will make Shalom with him and pay to him taxes and be his slave 1 . If not, it would be better not to inform that he will return, and Chizkiyah will cease to prepare for war!
Perhaps he was concerned for his reputation. He did not want people to think that he threatens and does not carry out! (PF)