The spine actually is surrounded by CSF (cerebalspinal fluid) In other words, fluid on the spine is really very normal. I know what you kind of mean though, the "disease" fluid on the spine, or water on the spine, usually relates to a herniated disc and has nothing to do with spinal fluid, but rather the soft tissue that should cushion each small bone in the spine from the next not doing it's job because of one disc rupturing, usually as a result of impact trauma.
i dont know the exact causes, ut ur right u spinal tap is needed usually between th 5th and 6th vertebrae, heres a link that might explain a bit more http://en.wikipedia.org/wiki/Syringomyelia
i don't know what causes it. but don't they have to " tap " into your spine with a needle and drain it ? and one wrong move could leave you parralized ?