This is exactly what horses with a pelvic fracture will do when a rider gets on. They can look almost perfectly sound as well in turnout and before you get on. I lost an older TB gelding who fell and broke his pelvis right in the middle at the bottom from this. He fell splay legged on a slick mat in the cross ties. I could have rehabbed him but he was 22 and lived to jump. He likely would not have ever jumped again and had a low chance of soundness on recovery due to age and bone brittleness, arthritis etc so I let him go.
"looking like a "frog" and like "a horse who was scared to get in a trailer" by that I believe they meant she looked like she was caving in on herself and was was up on her toes"