Spectacular is both a noun and an adjective. The noun spectacular refers to a big, beautiful production, like a play or musical performance that has a huge cast and many dance numbers.
As an adjective, spectacular has the same quality of being over-the-top, like a spectacular restaurant with excellent food, celebrities at all the table and a gigantic waterfall along one wall. If something is spectacular, it seems like a rare feat, a real stop-and-stare moment. That's why it can also be associated with drama, or making a spectacle or scene, like a spectacular argument you witness out on the street.