I remember seeing some old football clips in a special program that included some of the strange things that happened in games; miraculous catches, fumbles, hail-Mary plays that succeeded, and weird plays of all kinds. 
One of them featured a kickoff return. The receiver got the ball on his own 2 yard line and took off down the field dodging all the would-be tacklers. He moved over to the sidelines of the opposing team and was wide open for the rest of the field. A player, sitting on the bench, got up and ran to the side lines and stuck his foot out onto the field and tripped the runner! The runner went down but of course they called a penalty on the illegal player. Paul is calling a penalty on the Judaizers for doing the same thing. The Galatians were doing well. They were running with Grace all the way and on their way to a touchdown. Using the imagery of athletes as Paul loves to do, he says in Galatians 5:7, “You were running well. Who hindered you from obeying the truth?”
The Judaizers, snuck in, spied out the freedom and liberty that the Galatians enjoyed, and their loyalty to their traditions aroused a zeal that caused them to play in the game they had no right to play in. They tripped up the Galatians with the “leg of the law” and impeded their progress toward their goal. I’ve seen it in churches often. The church seems to be doing well; loving, learning, and living when legalism raises its ugly head. John Butler rightly observes in his “Analytical Bible Expositor.” He writes, “When a work of God begins to do well, the enemy of our souls will oppose it every way it can. When a church is going well, the enemy soon shows up to ‘hinder.’ The legalists (those who would mix law and grace in the Gospel) were very adversarial and opposed obedience to the Gospel, to Christ.”
Paul’s last question is “who hindered you from obeying the Gospel?” There is much confusion about this phrase, but let me simply say that to disobey the Gospel is to exchange salvation by grace for salvation by works. Richison says, “We get off the road of grace when we descend into the belief that we can impress God by what we do….Satan’s method is to get Christians to think they can impress God by their works. If he can get us to believe we can amass brownie points with God, he has us where he wants us.” It drives us back to our failures. Martyn Lloyd-Jones said, “I suppose there is nothing that so tends to rob us of our joy as our realization that we do not love him as we ought, because when we realize this, we become unhappy and miserable. I will tell you the best antidote to that: when you realize your love is weak and faint and poor and unworthy, stop thinking about your love, and realize that in spite of its poverty, he loves you. He has said, ‘As the Father hath loved me, so have I loved you’ (Jn 15:9).”