why have the leaves on my magnolia turned brown and look like they are dying

its getting late in the season and it is getting ready to shed them also we have had a dry spell and that also speeds up the process. nothing to worry about.
welcome to GoY too. :o)

Same thing happens to mine, it's pretty ugly right now! But it's fine.

Assuming yours isn't an evergreen type, its probably autumning, as previous comments say, and depending on where you live, this will have been exacerbated by the dry conditions (here in the south, anyway). Its not an evergreen one, is it?