It could be many things. Not to worry though!!
The things it could be are:
Saddle- fix through saddle fitting.
Back sore- fix through extra pad or by sore no more spray.
Hoof sensitive or soft hoofs- fix by turpentine or farrier visits or hoof boots.
Abscess- fix by soaking hoof in Epsom salt and then putting a mixture of sugar and bedidine in hoof and covering it in a diaper vet wrap and then ducktape.
These are all fixable solutions. I hope your bucking gets better! I know how it feels!! Lol!!