Particles are constantly randomly changing their state. Entangled particles change in tandem. As the GP said - to find out whether a particular change of state

Particles are constantly randomly changing their state. Entangled particles change in tandem. As the GP said - to find out whether a particular change of state was random or a signal, one would need to compare readings from both. Readings cannot be sent faster than the speed of light. Thus preserving causality.