What causes subduction?

When two tectonic plates come in contact and one slides under the other.

Typically oceanic plates will subdue under continental plated. Oceanic plates are usually comprised of Bassalt which is comprised of lots of iron which makes them very dense. On the other hand continental plates are usually made of granite which is silica based and is far less dense. So the denser bassalt slides underneath. A good example is the pacific plate being subducted under the south american plate.

When two plates meet, the heavier plate will be forced down beneath the lighter mass, as the pacific plate is subsiding beneath the North American plate.