The problem is, your co-worker was not doing anything illegal - he was just being a jerk. You do not have a guaranteed right under the law to be free from jerkdom. It was nice of HR to move you but legally they didn't have to do anything at all and there is no legal requirement that they not move you back.

Which means that even if you had proof positive that the SOLE reason for your termination was to prevent you from escalating the issue, it would not be illegal.