Looking at the evidences around the world I will say both, also the extinction did not happen overnight, it must have happened over a long period of time, dinosaurs and other lost species must have struggled to adapt or migrate during this time, the continous events must have been a major factor.

Also looking at the length of the volcanic eruption, i.e over seven hundred thousand years, it must have cause a significant change on the earth. If we give credit to the asteroid impact then we can also say the eruption aided the spread of debris in the atmosphere and for a longer time because as at the time of the impact, dating shows that eruption was already or also happening on earth.