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.