You mentioned the reason - something is not aligned. The cassette has some width and there's no way all the sprockets can align with either chainring, so, as you point out the chain isn't lined up and will tend to derail towards the center of the cassette.
To understand the problem better, take a look at the chainring's teeth and the cassettes. The chainring has pointed teeth that can pick up a chain even if it's coming from an angle. However cassette sprocket teeth are cut square, and are not good at picking up the chain if it comes from an angle. The cassette is 100% dependent on the chain being guided straight to the sprocket. They do this for faster more precise shifting, and the system works fine as long as the chain is moving forward, ie. from the RD jockey wheel to the sprocket, but it means that the rear will be prone to auto shifting if running backward.
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Last edited by FBinNY; 07-25-15 at 06:37 PM.