I realized that maybe I'm leaning forward when I ask her to slow down so I tried leaning back and that did help a little but she was still tripping. She never trips during a transition when I'm lunging her so I'm thinking it could be rider error too...
I think what it might be that she's trying to slow down as soon as I tell her to instead of gradually slowing (or more gradually, I like that she wants to stop but not that she wants to stop so quickly that she trips).
I think another big issue is that as soon as I ask for a downward transition she uses her front end to stop herself. She's off her forehand most of the time and she doesn't lean on my hands or anything but she does when slowing.
Any ideas on how to get her stop tripping? I bet it's a lack of muscle thing so what should I be doing to build those muscles?