hello!
Sorry to hear about you being bucked off. When you were out on the trail and the other horse wnet ahead and round the corner, was your horse then left at the front of the ride? It sounds to me like your horse is feeling insecure out on the trail, particularly when left by himslef or or frint so he has to make the decisions about where to go and what is or isn't scarey. This is backed up by the fact that he has his bucking fit when turned out by himself.
horses are herd animals, they need the security and company of other animals to be happy when turned out. Can you turn him out with other horses at all, or can you turn your goat out with him?
With regard to your hacking problem, firstly make sure that he is listening to your aids and is responsive in the school. Try hacking him out with others, firstly with him tucked behind them, then with him next to then, then with his nose infront until you can gradually get him going in front with him being calm. This will take some time as it is his confidence in you as his leader that you are trying to establish.
If you can turn him out for long periods with other horses, you will probably find that the problem may well disappear.
If you are still having trouble, I would recommend having your saddler up to check the fit of your tack. Horses often change shape, especially as yours is a youngster be will be muscling up and changing shape.
Good luck!
bye!