Why did my cat do this?

Because it's in her natural instincts. Cats in the wild would have to hunt birds and rodents as their prey so even domestics still have it in their instincts to kill. She would have brought it to you because your house is her territory and some people say it's a pride thing; she wanted to show you the good work she's been doing. You can't really prevent this if she has this killer instinct, all you could do is keep her indoors which would be cruel as she obviously enjoys the great outdoors so you will just have to make sure you clean it up or chase her out when she brings something in. Hope this helps :)