I don't claim to be an expert, but it's possible that your cat might see the area she can see outside the window as part of her territory (or at least a place she can potentially lay claim to) and want to inspect it more closely. My parents' cats growing up certainly saw the yard as their territory, but they were also allowed outside. I don't know whether a cat that was indoor-only would see the yard as her territory or not. My parents' current cat is indoor-only and clearly doesn't
see the yard as his territory, but my parents currently have feral cats living in the backyard that Maverick can see through the window. They've been there since before my parents got him, so he knew the backyard was was already "taken" from day 1.
I think I've read that not every area is claimed by any particular cat, that some are more neutral areas. I live in an apartment complex and we have a small colony of ferals living in the complex, as well as the cats of people who live in the complex occasionally wandering around. So I assume the complex area is considered more neutral instead of belonging to any one cat. Misty and Stormy certainly feel free to wander and explore on the occasions when I let them out (or more frequently, when Stormy makes a successful door-dart).
Also, cats are very smell-based creatures. I don't know if your kitty can smell all the various markings and territory boundaries from inside the house. She may not know who (if anyone) the yard belongs to and wants to go outside to check out the lay of the land.
Or, if you don't have a very high population of free-roaming cats in the area, your yard may just not belong to anyone, all ready for your kitty to stake her claim.
Like I said, I'm FAR from being an expert in cat territorial behavior, these are just things that are crossing my mind, so someone please tell me if I'm totally off-base. Interesting question, though!