a transform boundary is where the plates are passing each other like if you press your palms together but try sliding them at the same time. if you try that you will notice that the motion occurs in jumps rather than smoothly (pressing palms together as they slide). the plates move in jumps as well when the stress builds up past the bonds or friction holding the plates together. that is an earthquake when it happens at the plate boundary. other things can happen geologically as well as you might imagine, for instance normal faulting from pulling and even thrust faulting from compression effects because the plates aren’t completely smooth and sometimes get stuck in places. big rocks can be pulled apart and broken or crushed together and heated from the friction. within the transform fault will often be smaller strike slip faulting and is part of this, too. generally though, earthquakes will be the most common answer.