A horse will always work better for the main person who rides them... its just because of the bond one horse may have with it's owner/rider... but it should still work for another person in some manner. My best mates horse is AMAZING to ride, if you know the right buttons to push, and as much as he's so lovely to those who ride him, when his "Mummy" gets back up, he deals to her for putting someone else on him. My wee mare was backed and started by myself and when other people started hopping up on her she couldn't handle it at all... its was something her and I shared, no one else. But I persisted, and she finally realised that it doesn't matter who is in the saddle, she has to show respect to that person (she was never dirty, just wouldn't allow them to mount her without side stepping... as if to say "Hey! You're not Mummy!").. and they have to give respect back to her too.
I personally don't think a horse just suddenly turns "bronco" and does bucking and rearing in one session without a reason. He could have been geniunely scared, but it sounds to me he's sore. SO many people go "oh no, the saddle fits he can't be sore" (not saying that's what you're doing here)... but if my horse does something and I can't relate it to something I've personally done... I go through a list... are they sore? If they are, eliminate. The saddle could fit... but the muscles where the saddle sits could be sore... its very common that they can become sore like that. Has he taken a fall in the paddock when you haven't seen and put something out? Very likely... horses are good chiropractors themselves, every time they roll they realign their spine... but it doesn't take much for a niggle to become a problem.. most horses start off just lightly sore, and because they don't show us the signs, we continue on till it's really sore and they just have to go "OW!!! Get... off!" Of course, some horses react more dramatically than others.
If he's not sore, then is it a behaviour thing? Walking over eggshells isn't going to help, believe me I've had my fair share of sidestepping issues because I was scared with my gelding. I suggest if you're not confident, get some advice from someone more experienced, even on the ground. They could also determine whether Chinga is sore too.
Please don't just "give him a rest." It's important for the muscles to keep active, even if you just walk him. I've been taught by a professional chiro who has told me that's the best medicine... you stop those muscles, they get stiff and that's when the problem begins to mount higher... Walking is not only beneficial but will keep the muscles nice and loose.
I hope this is of some help to you... don't be discouraged love, it's alright to get help, even if your boy is a little disturbed by other people.
♥ Seoul Searchin' for the Lovebug ♥