John the Baptist was a mighty preacher. People from all over had flocked to hear him and to be baptized by him. Though he had baptized Jesus and witnessed of Him saying "Behold the Lamb of God, which taketh away the sin of the world" John 1:29, his personal ministry continued.
Jesus' ministry, which also involved baptism, was drawing ever larger crowds. The disciples of John the Baptist came to John with their concern. "Rabbi, he that was with thee beyond Jordan, to whom thou barest witness, behold, the same baptizeth, and all men come to him" John 3:26. I'm sure Satan would have been delighted if he could have incited jealousy in John's heart which could have lead to undermining the very work intended.