Final answer:
The theorized cause of massive extinctions at the K-T Boundary is a cataclysmic impact of a large meteorite or asteroid. This impact would have thrown a significant amount of dust into the atmosphere, disrupting the food chain and leading to the extinction of many species.
Step-by-step explanation:
The theorized cause of massive extinctions at the K-T Boundary, which marks the end of the Cretaceous period and the beginning of the Paleogene period, is a cataclysmic impact of a large meteorite or asteroid off the coast of the Yucatán Peninsula. This hypothesis is supported by a sharp spike in the levels of iridium, an element found in meteors, as well as the discovery of an appropriately aged and sized impact crater. The impact would have thrown a significant amount of dust into the atmosphere, causing a decrease in sunlight reaching the Earth's surface and disrupting the food chain.