If your skateboard trucks turn on their own, It's probably because you have been putting more weight on one side than the other. I would suggest loosening your trucks so they won't get stuck in one position.
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The axel which is on the truck could possibly bent. Another cause is that the wheels on your bard or one wheel might be coned and sort of flat. One other possible cause is that you could have upt the trucks on backwards, the part with the kingpin and bushings face the middle of the board.