Albert Barnes Commentary 2 Chronicles 35:24

"So his servants took him out of the chariot, and put him in the second chariot that he had, and brought him to Jerusalem; and he died, and was buried in the sepulchres of his fathers. And all Judah and Jerusalem mourned for Josiah." — 2 Chronicles 35:24 (ASV)

Josiah’s fate was unprecedented. Until this time, no king of Judah had fallen in battle, and none had left his land at the mercy of a foreign conqueror. This is why the mourning for him was so extraordinary .