The pharisees were probably sent to question him because he was practicing t’villah, or ritual immersion, outside of the confines of the local religious hierarchy, which was considered taboo at the time. But, he was the son of a former high priest. So, he was entitled to be part of the religious hierarchy, but was not, so that presented an obvious dilema for the local religious authorities, particularly given his rather miraculous conception and naming.
Ritual ablution, or t’villah, what we would call “baptism”, was not an unusual thing to the Jews at that time and it is still practiced today at Yom Kippur and for converts among the more conservative Jewish groups.
That John Baptist was an Elijah-esque wild man is without question, cf. Luke 1:17, John 1:21-25.