NEVER pay extra for the exit row
 

It seems the odds are very high that you will be stuck next to a terribly large person. I tried paying extra for the legroom and when I looked at the seatmap the day before the flight, I was the only one to choose this option on RDU-JFK. What I think happened is they took the 3 largest people at check in and placed them in the exit row. I can't think of any other explanation.

All I know is I had to lean over against the wall the entire flight and I couldn't touch my TV controls without asking him to raise his arm. :(

Hmm, never really noticed that trend, I'll be looking. On the E190 (which I presume is what you flew unless we had an a/c swap) row 12 is the only row with EML... We have 6 rows of it on our 320 (2-5, 10-11) so possibly more spaces for people to choose from.

How many times has it happened to you?

I bet that those seats weren't issued at check-in but were selected by the fat pax in your row. Why? Because if your fat, your uncomfortable on an airplane and if your not short 5'7 and below, 31" is a little bit cramped and the Exit row is the only row with EML on the E190. So they probably chose those seats so they could be comfortable. Unfortunately you were the person sitting next to them.

I've also had the 3 largest people onboard seated next to each other in non-EML and non-exit row seats as well! They all saw each other and all started laughing at the same time. It was completely random as the seats were assigned at check-in. We had room so it wasn't a big deal and I was able to sit them comfortably but it does have a random chance of happening.