Q: I am English and I don’t understand why Americans say “horseback riding.” In England, we just say “riding” or “horse riding.” It’s taken for granted that the back is the place of choice. I don’t know why it bothers me so much.
A: The term “horseback” is very old, and published references go back to the late 1300s, according to the Oxford English Dictionary. You can find it used as a noun, which sounds very odd to modern ears, and even as a verb. More commonly, it has been used as an adverb or adjective. Here’s an OED citation from 1390: “This knight, whiche hoved and abode Embuisshed upon horsebake.”
Now, to your question. The expressions “horseback ride” and “horseback riding,” according to the OED, are now “used chiefly in U.S.; in England, ride, riding are understood to be on horseback, unless otherwise expressed or implied, as ‘a ride in a wagon,’ ‘a bicycle ride.’ “
I haven’t found any reason for this difference in contemporary usage. It’s been my experience, though, that horsemen and horsewomen in the U.S. use “ride” and “riding” the same way they’re used in the U.K. Non-riders or occasional riders in the U.S. are more likely to refer to “horseback riding.”