I would say it's more common for people who fast travel a lot. I found when I used to fast travel, I would get a butt load of dragon attacks, but when I stopped fast traveling, it is more seldom now. Weird.
I would get two at a time often near riften stables when fast traveling on one file, always one at the college for another, with the riften file not so much after a time I only fast traveled part way and walked the rest. Even when I fast travel full way now to the stables, I have a lot of shouts and even more spare souls :/