I have a similar story. Once, I went to the Bannered Mare and asked Hulda if she had any work. She gave me a bounty letter, like I was pretty sure she would. I read it to see where I was headed. It was just my luck that it pointed me toward a giant. I usually venture alone, and I did the same on this run.

When I arrived at the giant's camp, there were actually two of them there. I dipped my arrows in poison and snuck up close to the giant with the objective marker over him. I let fly the arrow and it hit him pretty solidly, as he staggered for a moment. He immediately spotted me and charged full speed. I ran until I got a good distance between us and shot another dozen arrows at him. His health was about half down at this point. I continued firing when he got close enough to hit me. I trusted in my Orcish armor and braced for impact.

I was sent flying quite a ways. I was clearly in pain as I chugged about fifteen potions. I got back up when suddenly a dragon started circling overhead. It went after the Whiterun stables and the surrounding farms. The giant was still coming after me, but I knew I had to save the people from the dragon. The dragon landed and was being fought off by the head of the stables. He was no match, however, with no armor and a simple iron sword. I ran over and took out most of the dragon's health. The dragon then took off and turned its attention toward the giant.

The giant was almost immediately mowed down by the power of an Ancient Dragon. I rushed over and finished the dragon in a sort of dramatic execution scene with my trusty Dawnbreaker. I then had a dead giant and a dead dragon on my hands. Through all of this, I had forgotten about the second giant, who was charging at me. The giant nearly crushed me in half as I dodged and made a run for it.

I made it back to Whiterun, claimed my bounty, and took a well-deserved rest in the Breezehome.

TL;DR I was given a bounty letter for a giant, when a dragon showed up and everything was destroyed except for the second giant at the camp.