What do I do about a horse that charges?

I went to roundpen a paint mare named Cricket. She belongs to the owners where I board my horse. My friend, who is a beginner rider, is the one who always rides her. Cricket is always well mannered, gentle, and respects our space. My friend wanted to practice her roundpenning so I told her I would roundpen Cricket first to see how she does. I've never roundpenned this horse but I know that the owners' twelve year old grandaughter has. Cricket was very hesitant at first but I did get her to trot one circle. When I tried to get her to turn around she backed up and tried to kick me. I waited a few seconds and tried again and she did the same thing. So I backed off to let her have some space to calm down. I was standing in the center of the roundpen, just watching her and not doing anything, when I saw her ears flatten against her head. I thought 'uh oh' and then Cricket wheeled around, kind of did a half rear, and then charged at me. I ran to the edge of the roundpen (which I know was probably the worst thing to do because now she probably thinks she can get away with it, but at the time that was the first thing that came to my mind because I wasn't sure if she would stop or not). I was wondering why this horse, who was always so nice and gentle, would suddenly charge at me? I'm thinking she was probably scared, which is why I backed off to let her calm down in the first place after she tried to kick me, and that maybe that's why she charged at me. Or was she being malicious because she didn't want to work? I would please like to know what it was I did wrong and I appreciate your answers. Thanks!