This is an excellent article. I used to worry about my QH Cadillac Kent. He would trip in the arena and once even went to his knees. Then I took him on a trail ride where the terrains was uneven. Winding trails, lots of roots, with some steep areas. He was like a mountain goat. He was just bored with the arena and dragging his toes in the sand..
My horse stumbled for years and it was finally diagnosed as equine protozoal encephalitis. On our last ride, he fell all the way down. I wish the vet had thought to test him years ago. Maybe I'd still be able to ride him if he had been treated earlier. He is retired now die to muscle and nerve atrophy from "EPM".
Good article. In my horses case I have found 2 reasons. First, she is getting tired. Two time for a trim.
My horse is lazy! He does the same thing at turnout as a casual walk under saddle, but get him interested in a planned activity and he's a whole different horse!