Explain why volcanoes often form on the continental plate after an

ocean-continental plate collision.

Because when the tectonic plates collide, the overlapping of the continental plates lift up the oceanic plate, which therefore, forms the mountain or volcanoe.

In a nutshell:

Both the colliding tectonic plates, when an earthquake or continental drift occurs, collides against each other, and due to the equal force from both ends, the earth gets pushed upwards, to either form mountains, volcanoes or valleys.