“Jews” (KJV) is actually singular and should read “a Jew.”  A dispute arose about John’s baptism which either involved Jesus from the first or later came to, as indicated by the question of John’s disciples as posed to him in the following verse. RWP states “Probably some Jew resented John’s baptism of Jesus as implying impurity or that they were like Gentiles (cf. proselyte baptism).” However, the problem could have been over the lineage of the two baptists. John was of priestly lineage but Jesus was not.