My guess would be absolute luck. They'll think that it's because of their phenomenal skill controlling the bike. But, it's not. It is pure unadulterated luck.
It is, and I've seen what happens when that luck runs out first hand: its not pretty.
If you play poker, you know that even though you're heavily favored to win when you go all-in holding four of a kind, on rare occasions you will lose. A lot of people think its reasonable to bet your life on a 50,000:1 bet. Statistically speaking, more than 50,000 of them.
Even calling it bad luck is a bit disingenuous. If 100,000 people bet their lives on a 50,000:1 chance and two die, that's not bad luck. That's statistically uninteresting. But people are, when it comes to probability, complete idiots. They think 50,000:1 odds are so small
everyone should be able to beat that, or its bad luck when they don't. That's a critically flawed misunderstanding of how the universe works.
Many years ago near where I live there was an auto accident where a group of teenagers that were racing another car hit the side of an elevated highway, drove up and over the concrete barrier, sailed off the highway, through the air, and crashed into the parking lot of a gas station several hundred yards downroad, killing everyone in the car. They left the car there for weeks afterward as a form of public awareness campaign. Many people called the accident both "tragic" and "bad luck." Its not bad luck when you are driving so fast that your car has sufficient kinetic energy to propel it out of an elevated highway and a thousand feet through the air *after* leaving over a hundred feet of skid marks on the road. Thinking the cause of the accident was bad luck is why they keep happening.