It was an hour before sunset, but the holy city was already twilight dark. Late summer is Mecca’s stormy season. It’s also the season just before the hajj—Islam’s annual pilgrimage—and the Grand Mosque at the city’s center was already filled with visitors. A blustery storm was whipping through the city’s skyscrapers, half-built high rises, and thicket of tower cranes. It was probably a big gust that caught the large red and white crawler crane, pushing its massive boom like a giant lever until the machine did a backwards somersault and landed on the mosque’s roof.

When the crane hit at 5:20pm on September 11, it hit hard, and the collision shook loose tons of concrete and debris onto the pilgrims and visitors inside. One hundred and eleven are now dead, and nearly 400 injured. But cranes are for lifting big things off the ground—they’re engineered to be very, very stable. So how could one flip so disastrously?

“Their physics are pretty simple,” says Henry Petroski, an engineer who studies structural failure at Duke University. “Whatever you’re lifting is a significant force, and that has to be balanced by the crane’s geometry.” The crane that fell on the Grand Mosque was a crawler crane, with four basic geometrical parts: superstructure, boom, mast, and cables.

The superstructure, where the operator sits and where the crane pivots, rests on two tank-like treads. Attached to the front of that superstructure is the boom, the long arm that carries the crane’s load, which itself is connected by steel cables to a smaller structure—called the mast—extending from the back of the superstructure. The cables move the boom up and down, but the mast is what keeps the weight balanced.

Let’s say you want to lift 10 tons. That weight is pretty stable if it’s held close to, or directly above, the crane’s superstructure. As the crane extends the loaded boom, the superstructure’s base has more trouble staying on the ground, and the boom needs more support to keep from buckling.

Balancing all those forces comes down to counterweighting. Sometimes the mast is enough to disperse the load, but superstructures often get loaded down with huge concrete and steel weights to help. Crawler cranes also have extendable feet called outriggers that give it a wider base, for increased stabilization.

Despite their low center of gravity, crawler cranes—so-named because they move around on a pair of tank-like treads—are far less stable than teetering tower cranes. “Basically a crawler crane has to be on firm ground and level within one percent,” says Terry McGettigan, a 43-year veteran crane operator and proprietor of the crane safety site Towercranesupport.com. Soft ground will throw a crane out of balance, and an out of balance crane is ripe for disaster.

Wind is a crane’s greatest foe, and even a perfectly set-up structure is susceptible. This is because the boom acts like a giant lever that the wind can push upon. “If you think about it, the higher up you have the boom, the less wind it will take to push the crane over,” says McGettigan.

On the evening of the collapse, Mecca’s airport weather station showed sustained winds around 25 mph. This does not account for gusts—which could have been much higher—or how wind behaves when it encounters tall, clustered buildings like the ones that surround the Grand Mosque. “There can be channeling effects,” says Petroski. “Like, if you have water flowing in a stream and the channel narrows, the water will go much faster. That’s how you get rapids.” Tall buildings do the same thing, forcing the wind to go faster in order to go between them.

The toppled crane was also one of dozens surrounding the Grand Mosque. McGettigan thinks this crowd of cranes didn’t leave the toppled crane’s operator enough room to lower the boom. When the wind kicks up, crawler crane operators should either lower the boom down to the ground, or tie it down. “I looked at the pictures, and logistically it just did not seem possible,” he says.

Finally, the earth itself could have revolted against the crane. Cranes are incredibly heavy, and put an enormous amount of strain on the ground beneath them. “Rain could soften the ground on which the crane is sitting,” says Petroski. Even in very little wind, or none at all, the mushy ground could become the cause of an accident. Even properly seated outriggers wouldn’t be much help if they aren’t seated on solid ground. The images of the toppled crane appear to show the outriggers deployed.

Even the best engineers can’t control the weather, but they can try to accommodate it through safety procedures. “There are all these things that are supposed to be done,” says Petroski. “But you’ve got human beings involved so they don’t always follow the rules.” The shifty gust that caught this crawler off guard was devastating, but it was probably no act of god.