Then the men of Israel said to Gideon, Rule you over us, both you, and your son, and your son's son also; for you have saved us out of the hand of Midian.
Another contributing factor to this sin of Gideon might have been the latent hostility between him and the tribe of Ephraim mentioned in the first paragraph of this chapter.  The Tabernacle with its High Priest, etc. was located in Ephraim's territory, and Gideon might have felt the desire to have closer access to God than that of going through the Tabernacle at Shiloh.