You can file a grievance, but if they don't want to hire you they won't hire you. I applied and did all of the testing to be a police officer for 9 years, and couldn't get hired on. They kept telling me that I didn't have enough "life's experience". Then finally 9 years later an agency told me they wouldn't hire me because I applied to too many different agencies. I threw in the towel after that.
Now, keep in mind I am not a veteran, but I volunteered a lot of time with my local sheriff's office when I was younger.
IMO, it's politics. You probably didn't fit the exact criteria of what they wanted then. Try again. From what I have been told it is all about the timing. If you time it right then you will get hired on. Sounds more like luck to me. In any event, good luck and I hope you get hired on.