No one knows for sure but there are several highly plausible theories, which may have interacted with one another. But the really important point is that the dinosaurs disappeared at about the same time as many other species of animals and plants, at a point in geological history known as the K-T boundary. This was the time about 65 million years ago when the Cretaceous period (K) came to an end and the Tertiary period (T) began. Whatever killed off the dinosaurs almost certainly had a bearing on the rest of this mass extinction, which was global in scale and affected the flora and fauna in both the terrestrial and the marine environments. So whatever triggered the ultimate death of the dinosaurs, it must have been a pretty epic phenomenon affecting the entire globe for a very long period of time.