Ok, I went back and looked and it is uncommon recently, but (to my mind) staggeringly common on the whole.

Maximum Security did it. But then you have to go back to Super Saver, which was the last of 5 in a row. That streak was punctuated by Barbaro coming home almost 2 seconds faster - which was another race that slowed down dramatically in the middle fractions.

It doesn't seem like the kind of thing you'd expect in dirt routes. A couple of great Belmont horses have done it (Rags, and Afleet). It's perhaps worth noting that the Oaks this year did not fit that pattern - slowing consistently over the race.