usually This means your horse is not willing for the ride and knows whats coming. Or they are just sore and tense. At one of my barns we have some troubled horses just like that. To get rid of that habit we will just rub their girth area to loosen up tense spots and let them know its not the girth and do friendly game with the girth (rubbing it all over their body so they know it’s not a bad thing). And after this they don’t bother trying again.