I think your horse has figured out that he doesn't have to do 90 miles an hour all the time. That he can slow down and enjoy life so to speak. The best thing for any horse is to have multiple jobs in different disciplines. It is good for a horse that has an arena job to get out on the trail and learn to slow things down a bit.

I ride western normally but, Now that it is about 110 degrees here during the day, I have taken to riding in the late evenings and have taken my 27 year old trail horse and have started riding him in an english saddle and doing some arena exercises. He was not sure at first with such a light saddle on his back and so much contact with me. But I think he is enjoying it more than my western saddle.