I got myself into a disagreement on another thread recently so I need to try to make myself clear - I have nothing against desensitizing (other than its a really hard word to spell for some reason) Getting horses used to things by exposure is a good thing but it isnt a replacement for building up trust which is quite different. You flao a bag in front of a horses face enough times and it simply doesnt notice it any more but the same bag in a different situation may still freak it out. When a horse trusts you then its will do its best to believe in you and deal with pretty much anything that comes along.
The guy who said horses pick up on nerves is spot on. We had a mare once that couldnt be ridden by a nervous person as they freaked her out and yet she was a sensible boring ride for a confident rider and competed in dressage, showing & jumping really well. It made me realise that some horses are just more sensitive than others
My other thought is that 30 minutes of groundwork is quite a lot for a young horse to concentrate on and he might just have been saying 'stuff you I've had enough of this'
My pinto and our older Irish Draft both get a bit up themselves when they have some time off, especially when its cold so I do give them a short lunge session before I get on or I know I'll get some crow hopping and pinging around but the horse you have sounds more like he was really unhappy and fractious