Tectonic plate can be forced into one another. When at least one of the plates is oceanic, it will be force below the other plate in a process called subduction. These plate don't by one another easily. The results is the build up of tension in the rock layer that will, eventually, make a sudden shift that transfers energy ( motion) into the surrounding rock. It's an earthquake.

Where two tectonic plates meet they slide by one another. As they do, they become locked together and accumulate stress. Eventually, this stress is great enough that the rock slips and transfers energy into the surrounding rock. It's an earthquake.

Volcanoes can cause small earthquakes. Here magma is moving towards the surface. In an effort to do so it must shift the rock aside. In doing so, it transfers that energy into earth movement, or an earthquake. Scientist can often predict a coming volcanic eruption based on the frequency, depth, and magnitude of these earthquakes.