I might be able to shed some light on this, having read a brief note on the Racing Post website originating from Market Rasen’s Clerk of the Course which referred to a freak accident which had occurred.
So I’ve researched further, and it transpires he jumped into a brook and broke his back – I understand from the original note that the brook is part of their reservoir system and isn’t cordoned off by rails.
What an awful way to go – surviving a race mishap only to stray into a danger area whilst running loose. RIP poor unfortunate Amok.