I think the colic surgery could be a bigger factor than some people realise. Most horses would not come back from that to their previous physical peak. With a horse that has had his intestine operated on there is a much heightened risk of a recurrence, and it could prove fatal. As Freeradical already mentioned, perhaps the horse is just protecting himself.
Another thing to remember is that Montjeu’s are rarely brilliant over 10f. They generally need at least 12f to be at their best. Who knows what might happen over the longer distance next time out?