Interstates versus secondary roads

I was driving on I-20 several years ago and it was about 2 a.m. Traffic was very light. A guy with a badly misaligned headlight came up on me driving about 30 mph over the posted limit. He got right behind me and slowed and paced me. The misaligned headlight was shining directly in my rear view and was blinding me, so I sped up. He sped up. I slowed down to about five under the limit - he slowed down. I accelerated rapidly - he followed suit. Finally I just let off the gas and let my truck slow down. He finally pulled out and passed me when I got down to 40 mph. A half hour later I caught up with him again - he was tailgating another guy and doing about 10 under the limit.

I don't understand why someone does this. Miles of open road and they want to drive two car lengths off my bumper.