There are three possibilities for how:
1) He got too rambunctious by the stairs and fell. Again, I've raised kittens before, and in this house, and that's never happened. Even when Luna was a kitten, two weeks younger than Maki, she never did anything like that. Kittens have a fairly well-honed fear of heights, and it's a good ten feet from the bottom of the stairs to floor level. The railing is meant to guard against people falling down there, not cats. But again, years and years of cat ownership in this house and that's never happened before; I've never seen any cat, young or old, fall down there. (Doesn't mean it can't happen, of course.)
2) I noticed that a box in the bedroom had shifted. It's possible that he tried climbing on it, and somehow hit it just right so that it tipped over on him and got his leg. Narrow box, has books in it--maybe.
3) Maki has been jumping off our bed and onto the floor. He's done that several times a day since we got home Thursday. The bedroom has padded carpet; it's a soft landing. If Maki were to have somehow gotten onto the kitchen counter, and then jumped down from there--that's a hard floor. Ditto for the bathroom. Everywhere else is carpet. Hitting a hard floor the way he hit the bedroom floor coming off that bed would do it, too.
To make matters worse, the break is very close to the knee. They can't just splint it because of where it is; if it had been in the middle of the bone they could splint it and let him do whatever he wants. Animals are pretty resilient that way. But because of where the break is, the vet isn't sure what needs to be done, so they gave him some knockout gas and took a couple detailed x-rays, and e-mailed them to a veterinary orthopedist, who's going to look it over and come up with a recommendation.
Meanwhile, he's coming off the gas, and has been given a little pain reliever, so he's a little loopy. They advised me to keep him in a confined area, so he's in the big cat carrier with a small makeshift litter box. I gave him some food and water; he ate, purring loudly, then drank. After that I tossed in a sparkle ball; he played with it a bit, and now he's resting.
Regardless of how it happened, I'm really upset and sad about it. I feel guilty, too, of course, though I don't really know what I could have done to prevent this. Lock him in the bathroom? What if it was the bathroom counter he jumped from? The boxes? A seventeen pound adult Maine Coon routinely jumped on those very same boxes without them budging an inch--there's no reason for me to think they'd move when a half-pound kitten does! The stairs? As I said, in decades of cat ownership, never a problem.
I know it's just a stupid accident, but I can't help feeling like it's my fault. If it were possible I'd take the broken leg for him.