Crazy Mini-Van

I was heading North on I-75 and at this point there were 4 lanes to travel in.  I am the only vehicle in sight (its 2:45  am) when I see another vehicle's lights come over a hill behind me.  I am in the far right lane and they are one lane to my left.  As they come up to pass me I see them brake suddenly and pull in within 10' behind my trailer.  I know they are this close because I can see the glow from their lights, but can no longer see the vehicle itself.  Since I see an exit is coming up and I don't like to be tail gated I simply move one lane to the left leaving the right lane open for them.  This idiot mashes on the gas and passes me on the right, which I expected, but then he didn't take the exit!  Instead they continued on down the road, except now they are weaving from one side of their lane to the other.  

4 lanes of open road, we are the only two vehicles in sight and they HAVE to tailgate then pass me on the right?  I just don't get some people's stupidity!