Standing up too early on my iron swings
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straightening of the front knee earlier in the swing
Leaning away from the ball (reverse weight shift)
unhinging your wrist to early (not a flat left wrist at impact)
Usually these will cause thin shots
Rotate more with the body, use less hands in the shot. Try to maintain the front knee flexion more in the swing.
This is my guess, post a video of the swing when it happens.