I'm with Theroundpen - years ago we had a gelding who would randomly go lame while hacking... which resulted in somebody being made walk back. Without fail, as soon as we'd turn into the yard entrance he was as sound as anything. He was looked at by farriers and vets and nothing ever showed up.
If it was a one-off, she quite possibly had something annoying her. If it continues, have her looked at to elimate the possibility of a problem.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.