Jonathan Franzen is the first living novelist to grace Time magazine's cover in ten years. Halle Berry is the first black woman on Vogue's September cover since 1989 (and her over-40 peers are suddenly popular). Three possible explanations exist.
1. In an age of increasing media fragmentation and strong competition from the internet, print magazines are being forced to make bolder cover choices in order to get attention. The old taboos are going out the window as magazines search for new ways to draw readers in a crowded marketplace.
2. In an age of increasing media fragmentation and strong competition from the internet, nobody really cares about magazine covers any more. So magazine editors are just putting who they want on there, and not worrying about "the rules." Whatever's clever.
3. Statistical variations sometimes cause the appearance of "trends," which are in fact purely random.
[Number three is the explanation for most things in the world, though that won't sell any magazines.]