The closer one is to a body of energy (and energy's other forms: heat conduction, pressure, and viscosity), the slower one's rate of time is. It is the gradient of this gravitational time dilation (how much the rate of time slows based on a change in distance) that causes acceleration to occur.
From a Newtonian point of view (in a weak gravitational field, and where relativistic precession is all but none), the orbiting body maintains a constant rate of time, and so follows a circular path of constant distance from the gravitating body. I believe that this is also referred to as a "straight path in spacetime" results in a "curved path in space".
As for the actual mechanism behind gravitational time dilation, that's to do with quantum gravity (superstring theory, loop quantum gravity). I cannot go into detail on either, for lack of knowledge.