As Jeremiah left Jerusalem for the land of Benjamin, what did the guard captain Irijah accuse him of?

Answer: Deserting to the Chaldeans

At the Gate of Benjamin, Irijah seized Jeremiah, saying, "You are deserting to the Chaldeans!" Jeremiah denied it but was arrested.

“But when he reached the Gate of Benjamin, the captain of the guard, whose name was Irijah son of Shelemiah, the son of Hananiah, seized him and said, “You are deserting to the Chaldeans!””— Jeremiah 37:13 (BSB)