I'm sure your constant need to drive at 10mph pissed him off.
I actually have a bit of a lead foot and speed, but not excessively, on highways and interstates. I do, however, take 35 mph and less zones very literally as they are so low for the reasons of pedestrians and, more importantly, kids.
To more thoroughly explain what happened, I cam around a bend and was in the merge lane. He was on the road I was merging onto, and he had the right of way. I came around the bend at the speed limit of 45 mph and found myself dead even with him. I had a car in front of, who merged in front of this guy, and a car behind me, whom I lost sight of coming around the bend. I could not speed up to merge in front of him, as there was not enough acceleration lane to do so and it would have been dick of me anyway. I couldn't do a hard slowdown to get behind him, because of that car behind me. I had a couple of seconds of lane left at that speed and put my car in neutral and naturally slowed down to let the guy get ahead of me, but not screw the car behind me, who may or may not be trying to speed up for the merge. I merged behind the guy at the very end of the merge lane, with about five feet between us. I continued to slow down as I merged, so as to put distance between us and not be tailgating. As I did so, I realized the left lane was completely clear, so I put my signal on and safely merged into the left lane, where I could accelerate back up to the speed limit and passed the guy. As I passed him he flipped me off and said, "Fuck you asshole!" He then followed me into the parking lot, parked near me, and continued to yell at me, including the threats of kicking my ass.
I can only guess that he was set off by one or both of the following:
1) I came around the bend in the merge lane and was right alongside of him for a couple of seconds.
2) By slowly merging over at the end of the lane, I cut the distance close from my front end and his bumper.
As a physicist and mathematician, I calculated everything precisely in my mind to do as safe a merge as possible, given all of the variables. I still had to worry about the car behind me that was momentarily out of my rearview mirror. I didn't have time to relocate that car, so I chose the easing off the gas and going into neutral method, rather than a quicker application of the brakes. You'd have to ask him, but he must have thought this was offensive in some way.