Multi-star systems are actually pretty common: We think nearly half of sun-type stars have a companion. Some have two or more. But how does a crowd of stars form all at once?
All stars start their formation the same way: Gravity pulls a whole lot of dust and gas into a ball. In a single-star system, the denser clump at the center eventually collapses and starts fusion. But if the system is going to end up with multiple stars, something else has to happen first.
Or gravity can form clumps of material out in the disk of gas and dust that surrounds the core, the same way some gas giant planets form. Each clump's gravity pulls in other material, making it larger and increasing its gravity, and the process snowballs until there's enough mass for stars to ignite.
And these aren't just computer models anymore. We're finally getting telescopes powerful enough to see these formations in action, and so far the theories are holding up.