Bummer she was so bad that you felt her presence would ruin the upcoming trip. I'm sure you made the right decision for you.
A backpacking trip can easily alter to one's mood. If she was lagging behind, sounds like the trip was a challenge for her and that may have raised her overall crankiness level.
Conversely, others might be far more pleasant while doing something they love and being away from work or family pressures.
Personally, in group backpacking situations I'm more quiet and reserved than I would be in other contexts -- mainly cause I'm saving my breath to keep up. I can recall a moment on a group trip where I may have seemed rudely unresponsive to a fellow backpacker, but I just couldn't handle hiking uphill and talking at the same time.