Well, I am not sure scientifically how it is caused. But, I do get some backspin myself occasionally and there are two things I can tell you:
1. Clean Clubface - Make sure your club face or the face of your iron is clean. All the grooves pretty clean. The more the grooves get on it, the more spin it will have.
2. Hit it high - Trying to hit the ball high will sometimes produce a backspin or just a check up.
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