Motion, particularly the kind when you go up and down, causes some people to become sexually aroused.
It’s been happening to me since I was a kid. I also have to friends who get boners when they ride rollar coasters.
What is going on in the body to cause such a thing?
Excitement. It gets the blood flowing more quickly throughout your body, to all parts of your body. Why it’s enough to make you aroused probably also has to do with various chemicals that get released into your system when you’re excited for nonsexual reasons.
I saw pointed out once that, biochemically and physiologically speaking, all really intense emotions look pretty much alike. Your body will react much the same way to seeing a really hot member of the opposite sex as to seeing a smilodon about to pounce on you. The difference is all in the context.
Another vote for “it’s not the movement, it’s the adrenaline”. You’re getting combat hots.
Confession time:
This sometimes happens to me on the bumpy shuttle bus between work sites, so I don’t think it’s emotion related, unless boredom counts.
Perhaps it’s stimulation of the prostate?
Women who have ridden motorcycles, or sometimes horses, are giving you all little secret smiles. 
bicycles and even treadle sewing machines too.
it just ain’t fair.