I'm also thinking that the horses were feeding off on the "energy" the riders were putting out. I've ridden my horses in all sorts of weather and never noticed any change in their demeanor, but then I was calm even when the thunder and lightening started on one ride up in the mountains (not my favorite memory).
If you ever watch your horse out in the field grazing and it starts to rain and thunder ect...chances are it won't faze him/her. At least it doesn't faze mine.
I ride mine at all different speeds along the trail, and also on the way home. Mostly to find where our communication/training may be weak. That way when something like a change in the weather does occur and I put him/her into an extended trot or canter it's just par for the course.
Some of my favorite rides have been in the rain. Fewer bugs and in good rain gear it is very relaxing and I love the smells along the trail.