Yeah ... What he said.
Some rule outs might be: cerebellar hypoplasia, peripheral neuropathy, vestibular disorder, demyelination/myelination deficit, neurotransmitter derangement, just 'cos they feel like it, etc., etc.. (I'm tired and some of those might be way off, so feel free to rip those apart as you see fit ...)
Could still be a wicked cool science fair project tho.