When two tectonic plates push towards each other, one continental, and another oceanic, the oceanic plate is subducting under the continental plate. This is due to the fact that the oceanic plates's crust is weaker and has lesser density, while the continental is stronger and has bigger density, so when pushing against each other, the weaker, less dense, oceanic plate can not oppose the stronger, more dense, continental plate and slowly subducts under it.