1. Abrupt changes in behavior mean there has been an abrupt change in the horse's health.

There is a slight possibility he just hated that trainers ring or had a little bug or something...but that is usually wishful thinking.

Alternative therapies and chiro are great but, instead of throwing money at what might be wrong and hoping it helps, why not start by calling your vet and getting a workup?

This horse is 10 and has been working most of his adult life. Arthritic changes really kick in around that age and his hocks are a likely culprit. Even if he's back on track tonight, watch him carefully. If he starts loving the long spot, refusing to wait to get deep, rock back and power off the hocks, diving to one side at the base or skipping behind on his changes-look to the hocks.

A variety of other things may be bothering him up to and including his eyes.

I suppose it's possible he was just being lazy and in need of a stronger ride but ones that suddenly start slamming on the breaks usually have issues that need to be identified and dealt with.

What kind of maintenance program do you have him on for joints? How long since he's had a good health check? Lameness works up? X ray