I had a similar experience a little while ago. I had a girl in my bedroom but when I made my inuendo's less subtle she suddenly left. (I was not really in for a lot of foreplay so I became very direct)
Off course it could have been that she saw something she didn't like

, but I think it's about comfort. If you are pushing too much she feels overpowered. Also, if having sex iin bushes goes to easy for you, it might look like it's a habit..
I wouldn't feel bad about it. Next time maybe you can ask her to undress or something. It's very important you don't turn into a hungry predator once you feel like you're in.