First a little bit of background.... I have had this horse for 3 years now. She is now 20. She's a quarter horse, named Lassie. She showed 4H in her younger years, has done some team penning (very slow team penning but great for beginners), wonderful on the trail, we take her swimming, we take her everywhere. Has no vices, is pretty much the perfect bombproof horse. Anyone can ride her, and we always put complete beginners on her and she just carries them around. I bought her for my younger sister to learn to ride on, who is still a beginner.
So this spring... we have ridden her occasionally over the winter, and this is not uncommon at all for her to go for a while without being ridden, and she is always perfect coming back into work. We go for a ride. I am on my other horse, and Lassie is being ridden by a novice friend. She bolts! As soon as you bring her to the lope, she's gone, just takes off. So my friend falls off, gets back on and lassie is still being a pain. When I ride her, she is a complete doll, when my dad rides her, again a complete doll, he even rides her without a bridle and saddle. Now when someone else rides her, she will bolt with them. I am a little lost as to what to do. I figured I could have friends over who I know could handle it, so when she tries it with them they can correct it, since she doesn't do it with me. I can't put beginner riders on her now, and I just don't understand why she is acting like this all of a sudden. I was thinking maybe some sort of pain, but if that was the case I would assume she would do that with me and my dad as well. Any ideas on the cause and way to fix this?
A picture of the sweet gone crazy thing