I found a burning house south-east of riften.

Why is it burning? Is there any quests to do about it?

Found the same thing and was wondering it myself.

Dragon attack?

Look around in the house.

I originally thought it was the house that Astrid takes you to in the beginning of the DB questline, i.e. she burned it down after you gained entry. However, I later took an arrow in the knee discovered that house again with the two people I hadn't killed still bound and awaiting execution.

There's a circle of candles and a burned person with a "Summon Flame Atronach" scroll.

Remember kids, summoning is a dangerous art. The daedra will just as gladly kill you as the one you want them to

I found that a couple of days ago, and my initial reaction was 'Dragon attacked here'; I had gotten a notification earlier about the one that roosts south of town on a word wall, and assumed that it (as they sometimes do) swooped down and went on a rampage. Seeing a toasted corpse only reinforced this, but then I saw the scroll and figured it out. Wish said Atronach had still been there, although that would have been a dead giveaway.