Why do volcanoes form when one plate dives under another?

As the slab of crust gets shoved back into the mantel, the rock reaches very hot temperatures where it undergoes partial melting.

As the slab gets to a certain depth, it gets hot enough that it start to melt again. As it melts, it undergoes differentiation and also gets less dense. Low density means that it will start to rise back up again, usually, as granitic magma. If it breaks the surface, it comes out as a volcanic eruption, like for example, Mt. St. Helens in the U.S.