Great question — answering as the UphillTech team:
When I saw your question my first reaction was, you kept the yarn to tight between the stitches taken with it, which makes that the different sections in that one colour are pulled towards each other. On the other hand, if you leave your yarn at the back to loose it might get knotted and as such begin to bunch up, but this is much less likely. If you can face a bit of extra work, try knitting a test piece where you use only two colours and test out how tight the yarn needs to be in the back. Working with many colours will make the matters worse if you make mistakes with the tension.
Full answer by Willeke: https://crafts.stackexchange.com/a/11366 (CC BY-SA 4.0).
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