Well, maybe the vegans more often ride a bicycle than the other group because they care more about the environment.
EDIT: also have a look at the numbers: 65,000 people of which "nearly 2,000" were vegans, so only 3% of the sample. And they found "20 more cases per 1,000 people", so an increase by 2%. The study is likely to have an error margin of more than 3%. And the study "did not account for poor bone health or accidents".