I've been playing this game quite a bit and never once had a dragon attack inside a walled town. I was starting to think it was all heresey. Now in the game I'm currently playing, I had 2 different dragons attack Solitude twice...back to back! I love it when dragons attack towns. That's the point of this whole damn game!
I believe this to be a glitch. Hear me out. You know how when you walk by and a random NPC says something to you, and if he/she keeps talking while you go through a door, they follow you inside?
Well, that thing happened to me with a Dragon in Windhelm. I was just activating the door while I heard his wings flapping. And he followed me inside. I even tested this with a reload. If I wait a few seconds after the Dragon spawns, then I enter the city, he'll stay outside, but if I do what I originally did, FT to stables, sprint to the gate and activate it at just the right time, BAM! he comes after me!
About OP. I can too say that FT makes the Dragons spawn more frequently. If I FT I get a lot of them(on almost every FT to a random dragon spawn point - yeah, they're not so 'random'), but on one of my characters with no FT at all, 150 hours+, I got like maybe 4-5 random dragons.