Why does sunburn cause fever?
Here is a good article on the topic.

https://www.nlm.nih.gov/medlineplus/ency/article/003227.htm

But it's most likely due to the fact that a sun burn is an actual burn on the skin that can cause inflammation, inflammation can in turn cause fever. Also having a really bad sunburn can open you up more to the possibilities of skin infections. If this happens then once again you might get a fever due to infection. If you really want to find out more on the cause of fever after sunburn you need to examine the pathophysiology of fever and why fever happens. I bet my money on fever due to skin inflammation after a sunburn, I guess the real question would be why does inflammation cause fever since sunburn = skin inflammation

Here is a good article

http://antranik.org/inflammation-and-the-pathophysiology-of-fever/