As one plate subducts beneath the other, there is an incredible amount of friction between the two. The plates sort of "lock" into each other, while each is still moving toward each other. Eventually, something has to give way, and that "lock" breaks. This locked zone then becomes the rupture zone (also known as the hypocenter of the earthquake). This friction gets converted from potential to kinetic energy when the plates slip, releasing a large amount of energy.
2)The pressure of the plates rubbing against one another
Transform boundaries are where plates slide past one another. Also called faults, they usually happen where two ocean or two continental plates meet. They are most common on the ocean floor. These plates are colliding with huge force, causing one to slowly grind over, under or alongside the other. As the brittle crust gives way under the pressure, a fault ruptures and an earthquake is unleashed. The pressure of these plates rubbing against one another often causes frequent earthquakes that range from mild to severe.