The above represents the type of underwear I think she was wearing. Apparently, these being her favorite brand and style, she had worn them to the point of becoming brittle and when she pulled on them, the side piece tore.
Knowing she had little options, sitting there as she was, she proceeded to tie the broken pieces together. I think the resulting scenario would look something like this MS Paint sketch:
As you can see in the repaired sketch, it appears that approximately 1/2 of one buttcheek would be "free" for the rest of the day. Also, due to the quantity of material needed to "tie" them back together, the resultant panties were a little bit small. In fact, fearing she would never be able to get them back down or up, this trip was the last to the bathroom for the entire day until she returned home.
Obviously, feeling uncomfortable, sure that someone could tell and needing some resolve from outside herself, she emailed her sister to get some feedback. The response:
"Sometimes things happen. I just saw a cow mounting an alpaca and the alpaca looked very uncomfortable."
So ladies and gentleman, I guess that your mom was right. Always wear a good clean pair of underwear when you leave the house.