Usually the answer is "<Shrug> I dunno. The back of my brain I guess." Story ideas pop up out of nowhere when an something in the world suddenly connects with something in my subconscious to become an image and a character and a concept. It can be a phrase in the litany at church, the color of sunlight on a leaf, a daydream. It can be a personal frustration that I want to work out on paper.
Writer's brains are like compost piles full of books, images, sounds, scents, a half heard conversation, the look on a stranger's face, and all the questions we ask ourselves in the dark at 3am, or on the bus ride downtown. I probably say to myself "there's a story in that" a dozen times in a day, but I never know which seeds will germinate and grow into a story.
But I do have a slightly better, or at least a more concrete answer for this particular story. Like the protagonist, I actually do hike the Glendora Mountains. I'm fascinated by the thought of running into one of the mountain lions that everyone else seems to have seen already. (It seems that every other hike I meet someone who has a story to tell about seeing one.)
So far, all I've seen are tracks like this one; the distinct M shape tells you it's a cat, not a dog track. (That print was about the size of my palm.)
I love the outdoors. I'm an urban comfort sort of girl, most of the time. Give me your Asian fusion cuisine, your antique shops, your obscure booksellers' shelves, groaning with ancient tomes. But there is still a part of me that needs the outdoors; somehow it reconnects me to myself, puts the frazzled pieces back together into a saner whole. Problems get solved, ideas coalesce and swim to the surface.
So this is where one of my ideas came from. Can't you just see the sabertooth? Don't you want to?