It's quite hard to say exactly why dogs do what they do. Since they cannot talk to tell us, we can only basically guess why.
I would say there are several possible reasons. For one, your dog may just like to roll and rub her face in the leaves - like the way it feels, smells, etc - just because it's an enjoyable thing to do. Or your dog's face may be itchy and she's rubbing in the leaves to relieve the itchiness. Or you dog is rubbing as a dog would when rolling or rubbing on manure, a dead fish or other gross musty smelly thing. One of my dogs would roll on a tiny dead insect or an earthworm. I'm sure that the ground and leaves smell musty. Rolling in smelly or musty items is possibly a form of scent marking or of covering up their own scent (a sort of doggy perfume).
Hope this answer helped!
Can I be of additional assistance? If my answer to your question about your dog rubbing her face on the ground was helpful, please click on ACCEPT so I can get credit for my work. Thanks!