I think you had too many exciting elements going on at once (which was definitely not your fault)! The weather alone can be enough, as my mare is all too happy to show me ;) I think you handled the situation really well. I like that you kept him working until he relaxed (even if it's because he was tired). You ended on a good note, and that is what he'll remember. In answer to your questions:
1) Is it normal for a horse to all-out buck on the lunge and never do it with a ride? "Never" may be too strong of a word, but I think they're less likely to do it when you're on their back. I think horses start to associate lunge time as "play time" and riding time as "work time", so you start seeing more of a difference between the two.
2) Did seeing the other TBs get crazy cause him to "follow the herd" and spin him up? I think it was a combination of everything. He was feeling fresh, the weather was nice, and all the other horses were having fun! He wanted to play too!
3) What means he wont do this on Sunday? Well, you're on his back this time to help channel his focus, and he'll probably have been worked more recently so he has less pent up energy.
Also, I think horses have bigger reactions to changes in their home environment. They're there everyday, they're used to everything the way it is, so when something changes or is exciting, it's a huge deal in their minds. Yet, when you take them away from all of that and put them in a strange environment, everything is new! They're not going to spook at 'the horses that are there today but weren't there yesterday', because they weren't there yesterday to know the difference. They may look around and things that are bright and big, but I they seem to be disoriented enought that instead of throwing fits, they turn to the rider for direction.
That last paragraph might sound crazy, and maybe it is. I'm not basing this off a study, just what I've noticed in my own showing experience. Maybe it's true for other people.