Wikipedia can do a far better job at describing this tragedy. So I'm going to stick to what the former nuclear engineer in me finds interesting about this event: It was entirely due to incorrect engineering. The original design called for two walkways to be hung from the ceiling of a four-story hotel atrium. The walkways were to be suspended from a set of 1.25-inch-diameter (32 mm) steel hanger rods, with the first-floor walkway hanging directly beneath the fourth-floor walkway. 

The key to the design (and subsequent collapse) was that both walkways were to be suspended from the same set of threaded steel rods. Sometime prior to construction the builder complained that having fully threaded steel rods supporting both walkways would be an issue, as the threads would likely be damaged during construction. They asked that instead there be two separate set of rods, the first set suspended from the ceiling carrying the fourth-floor walkway and the second set that ran from the fourth floor walkway to the first-floor walkway. 

While this design change may appear to be minor with the rods connected to the ceiling still carrying the same exact load and the rods connected to the first-floor walkway only carrying half as much, the problem was that the structure of the fourth-floor walkway now had to support twice the load it was designed for: both the fourth-floor walkway and the first-floor walkway.  Since the fourth-floor walkway was designed with a safety margin, this wasn't an issue until the hotel was holding a 1,600 person tea dance on July 17, 1981.    

The disaster exceeded the capacity of the Kansas City Fire Department, directly effected over half the city's residents, and pitted the city's two major companies against each other: Hallmark Cards (the owner of the hotel) vs. the Kansas City Star (who won a Pulitzer for the immediate coverage and the subsequent detailed investigation).

The owner of the engineering company and an engineer of record for the Hyatt project, lectured at engineering conferences for years after the tragedy. Claiming full responsibility and disturbed by his memories "365 days a year", he said he wanted "to scare the daylights out of them" in the hope of preventing future mistakes.