Nothing on my character even slightly resembled a Freelancer affiliation. I was using a republican shipping ID, had no IFF and was indeed flying a Behemoth.
As displayed here, carrying a Republican Shipping ID allows you to pirate corporate vessels belonging to houses at war with Rheinland. You were flying a DSE IDd transport, which is a corporation affiliated with Liberty, which is at war with Rheinland.
It was perfectly within the boundaries of the rules to pirate you.