He made them fall out with black magic then felt guilty. Only rational explanation.

How about: The buspassenger 'pickpockets' the keys from the biker as they are talking, then has an assistant quickly drive back with those keys and dumps them on the street just before the biker arrives.
I think that makes a lot more sense.

I think the bus guy was just staring at bike guy's ass.

Easy, the bus guy entered sneak mode, pressed the use button to pickpocket once the biker was close enough. While he looked through the biker's inventory and took probably his gold and maybe some necklaces and rings, he accidentally took the keys. Upon realising his mistake he tried to reverse-pickpocket the keys, got the wrong button and droppen them instead. To gain something from the whole botched thing, the bus dude told the biker in the hope to level his speech skill.
He probably just playing some Thief Guild mission now, forging some ledgers. Unless he's with the Dark Brotherhood. Then he tried to poison the biker.

I got so distracted by your weird answers that I failed to realize what actually happened. I thought the guy saw the keys fall from a mile away, when actually he just saw them fall when his bus was passing the biker earlier. Not nearly as impressive.

I don't understand the excitement.

The bus passed the biker. While passing the passenger happened to notice the keys fall out. The biker caught up to the bus. The passenger told the biker.

WOW! MYSTERIOUS!

nah, not so much

It's possible because -- fake.

yeah! the keys are props of the passenger! he's practising for the Tour de France for his country.

Totally agree with you. I'm glad you agree with me. This is true harmony. I love you. <3