My guess would be that her back was
hurting her, particularly since she was tracking crooked. And don't feel
bad about overlooking back problems. As far as I remember, there were no
horse chiropractors 30 years ago, so there probably wasn't much that could have
been done for her at that time.
I had a rearing horse given to me 30 years ago, and she was a
sweetheart on the ground. She just never tracked straight under saddle and
reared often. Now, after all these years, I find myself wondering if she had
back injury/problem that might have been fixed. Wish I would have checked.
Oh, well.