a) Because of an increase in temperature, such as when continental crust is melted by hot basalt rising from the mantle.

b) Because of a decrease in pressure, such as when extension causes the crust to thin, reducing the pressure on the underlying mantle, or when masses of the mantle diapirically rise as a "hot spot". This is very common.

c) Because of the addition of a "flux", stuff that lowers the melting temperature of other material, such as when water is released from a descending oceanic slab into the overlying mantle wedge. This is very common.

They’re buoyant! They’re 10% less dense than the rocks they melt from and thus rise.

a. they’re buoyant

b. Lava is carried out of a volcano's vent as gases are released explosively, like carbon dioxide gas escaping from champagne.