Based on human chiro experiences I'm going to say that while he may have been in some degree of pain and unable to collect, he was in the chronic phase and had learned to deal with his discomfort. It sounds like now he's been adjusted, then he's fractious on the trail where he's never been, maybe he's feeling better but not trained to trail riding. Sorry, haven't read all the other answers, if I duplicate anything.
I would A/ go back to ground work, side reins, lunging and cavaletti work. B/ Re-introduce him to a DIFFERENT, not harsher, bit and teach him to seek his own relief from the bit. C/ Take him out on trail as often as you can but not just for short ride, go miles and miles until he is tired out. Walk only, nothing faster. Once he's nice and tired, then trot a little. Keep this up until he is even more tired. Keep riding him like this and once he has gotten a bunch of miles on him and learned to deal with all the trail obstacles and surprises, then you can start letting him out a little bit at a time.