Why do cars behind me flash their brights when I'm in the left lane?

OK, so I was driving on Interstate 95 in Georgia today, there were three lanes of traffic in each direction. The speed limit on this section was 70 mph I was in the far left lane, as I generally like to travel around 78 mph. I usually stick to the middle lane but was in the far left because I was passing a couple vehicles. Anyway, there was a car flying up behind me flashing their brights at me as it was approaching. I don't know what they wanted me to do, because I was in the process of passing several vehicles. I maintained my speed for awhile, and several times they kept flashing me and were tailgating me. At this point, they had thoroughly pissed me off so I stayed in the left lane even after I passed the slow cars. Finally, they passed me on the right, and got back in front of me and slowed down. I went to pass them on the right and then they sped off into the distance. What I was wondering is, who was in the wrong? I think it's just plain rude to flash lights at the car in front of you, it's not a good idea to do that you really want them out of your way. It just pisses off people like me. I'm sorry, but I was going almost 80 mph, which is plenty fast enough for an interstate.