i have that same chair!
we only have one though, as a work chair. its totally beat up and has some cracks of its own. i am in the process of stripping it. the wood underneath is wonderful., is the crack a stability problem, or just in the top veneer? i dont think there is anything you can do combat the wear though.

The horizontal, shard-like crack is in that outer layer, and the vertical crack on the side is, maybe more accurately, a separation of layers. The 'give' on the back of the chair feels different than on the others, and there's a slight sound, so it's definitely a structural issue. I'm not concerned with the normal wear-and-tear, only with what just happened. Maybe I'm paranoid, but I feel like this chair is suddenly garbage and, while that's frustrating, I'd love to know if there's any way to maintain the others to prevent this from happening again.

This is a known design flaw of these chairs, but it's not all that common, since you have to really lean hard for this to happen.

I own one single Fritz Hansen 1968-issue original, made in Denmark, and it now sits in a corner looking pretty because I've already cracked it (exactly as you did, via a houseguest). It's still usable, but any forceful lean backwards and I can tell that it would get worse. Mine is barely visible via a tiny hairline fracture.

There's not much you can do about it because it is inherent to the design/ materials. It's difficult to strengthen the back without altering the chair's appearance.

For your other chairs, I would suggest that if you live in certain climates/ region, to use a humidifier and not let your place get too dry. When you have dinner parties, you should sit on the cracked one, and remember not to lean back too much.