Why was I given Dragon Scale Armor?

Solution 1:

Dragonscale Armor shows up in leveled lists when you hit 46, so any quest reward or other event (such as one of the random encounters) that give you an award from such a list might generate a piece, although it's quite rare.

The Stormblade title is bestowed upon you when you complete The Battle for Fort Snowhawk. While the reward is listed as "None" on the wiki, discussion suggests that you will get some form of leveled armor as a reward. It looks like this comes from the concurrent, overarching civil war quest (the "leveled cuirass" reward).

In other words, it looks like Ulfric awards you some random leveled loot for finishing the quest, along with the title, and you happened to get lucky enough to roll some Dragonscale.