Roll downs

I have Birmingham rollers. I have a kit of about 20 birds and
one of my best flyers recently started rolling down. I have had this bird for two years and
she has never done this before. Luckly she lands in a field of tall grass and hasn't hurt herself. She has done this the last three times I have flown her. Does any body know what might have caused her to do this? I have not flown her since.