My TWH 7 yr old mare had a similar experience. She is only 14.3 and she cleared a 5’4″ fence from a stand still !!! This a mare from a family farm, 20 acres of fields, imprinted, trailered and trail ridden a lot. At the training farm she had hay 24/7 and was out all day, with her sister who was also there and loved it. At first we thought it was hormones and put her on regimate….no go. Finally my vet suggested an ulcer test and it showed she had both front and hind ulcers. A horse with food 24/7 ??? So it was the standard Ulcer Guard I found Smart Pak the cheapest and fastest for that. For a while I had her on Smart Pak Ulcer stuff that is like a tums. It helped but I’m not sure its a good idea for ever. Not sure……I consulted a horse nutritionist and she recommended Starting Gate which is a pectin and lethicin combo. It is suppose to work at the cellar level. She recovered. But I can assure you that we saw the most bizzar behavior from her and it would come and go. One day we were riding in a field, as soon as we started down a hill she got squrreleeeeeeee. Maybe her guts slide and set off an ulcer pain???? I agree with all of the other ideas on checking out the stable, food, water etc. But I can tell you that ULCERS can sure make them have very crazy and very unpredictable behavior. I use Ulcer Guard for a few days before I take her anywhere !!! Good luck !!!