Dinosaur extinction still remains one of the greatest enigmas of the history of our planet. Anyway, the most likely cause of the dinosaur extinction is climate change brought on by both extrinsic and intrinsic events.

The extrinsic event appears to be meteor as evidenced the crater at Chicxulub, on the Yucatan Peninsula of Mexico which caused massive terrestrial disturbance, which threw up soot into the air, causing short term acid rain, emission of poisonous gases, and cooling.

The intrinsic event is huge volcanic evidenced in India during the Late Cretaceous, eruptions were spewing forth floods of lava which can be seen today at the K-T boundary (these ruptures in the Earth's surface are called the Deccan traps).

This might be interesting: Geologists are probably evenly split between the two causal events but both are most likely root cause of the mass extinction event that included dinosaurs.

More Info: The Alvarez Hypothesis and both interinsic gradualists and extrinsic catastrophists !