This is truly sad, and just goes to show how fragile horses are. He was galloping across open ground, just as if he had been galloping in his field at home, stepped on some bad ground, and shattered his pastern. Could happen to any of us, and, in fact, happened to me when I got out of bed with a leg that had gone to sleep and twisted my foot funny. Just broke a bone from stepping wrong.
The connection to Colin Davidson makes it even sadder because his mother won’t have Crackers around as a living memorial to her son.