A.T. Robertson Commentary John 19:21

"The chief priests of the Jews therefore said to Pilate, Write not, The King of the Jews; but that he said, I am King of the Jews." — John 19:21 (ASV)

But that he said (αλλ' οτ εκεινος ειπεν). The chief priests were uneasy for fear that the joke in the mock title was on them instead of on Jesus. They were right in their fear.