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This is likely a set up and technique related issue. These kinds of issues tend to happen in people who pull with a flexed spine and then fixate the bar overhead while they back is in over extension. There can be a lot that leads to this but start with making sure you understand how to brace your spine from the beginning and keep it in a good position the whole time. This probably means going back to squats and overhead squats and looking at how you do them. Then you can go see whats happening at the shoulders and upper back and hips and ankles. The snatch requires a tremendous amount of skill, mobility, and motor control. Many people do them because that is what the gym says to do when they should really be modifying the movement.