Your bond together clearly shines through! Has he stopped bucking whilst being ridden? I dont mean to sound patronising but it could be pain related, if he is still bucking it might be worth getting him checked over. If he has stopped or if you have had him checked then I guess it was just down to naughtiness.
As I said I can see you have a clear bond with this cheeky chap and its lovely to see x
ETA: its lovely to see a healthy native living naturally in the field and not rugged up to the eyeballs! Lovely boy and a good doer which is lovely to see. He looks like a gem! x
What a naughty boy, sorry but that did make me chuckle as I've got one that behaves the same when the mood takes him. Well done you for sticking with him.
When he'sgood he's very,very good, when he's bad he's horrid!