What causes pulseless ventricular tachycardia?

Not really pulseless. When VT occurs at a fast enough rate, there is inadequate time for ventricular filling, cardiac output falls, and blood pressure falls. You may not be able to palpate a pulse but an arterial line would show a waveform that is reduced in amplitude. Rate is the key. As tissue perfusion fails, acidosis, hypoxemia and hyperkalemia result leading to ventricular fibrillation and death.