Green’s Dictionary of Slang

a fall (from horseback) onto the top of one’s head; sometimes ext. to refer to the fallen rider.

1861		G.J Whyte-Melville Good for Nothing II 201: A ‘crowner’ for John, whose horse goes shoulder deep into a hole.
1879		in Daily News 28 June 5/7: The inevitable fate of the rider is an imperial crowner, with, as like as not, his horse on the top of him.
1886		M.E. Kennard Girl in the Brown Habit I 177: My immediate follower had become an imperial crowner, and his long limbs were measuring mother earth.