Are there more causes besides Gravity and Velocity that cause time dilation? Albert Einstein discovered that both gravity and velocity alter time in a given reference frame relative to the time of an observer in a different reference frame. If gravity and velocity both alter the passage of time, is it possible that other external forces can alter time also? If so, what are these other causes?
 A: The answer is no, according to what we know today, there are no other causes.
For the answer you have to remember what is time dilation: time dilation is the dilation of the absolute proper time of particles from the point of view of an observer. That means that each particle/ each worldline consists of absolute spacetime intervals $ds$ which are corresponding to the proper time of the particle. The value of $ds$ is not an observed value and does not appear directly in spacetime manifolds such as the Minkowski diagram, it can only be calculated from the observed coordinate time and the observed displacement. In spacetime manifolds time appears in relative form, i.e. in the form of coordinate time. Coordinate time is proper time after time dilation.
Coordinate time and time dilation are dependent of the coordinate system that you chose, and by consequence, they depend on the relative spacetime manifold of the observer. In order to know what kind of time dilation may exist it might be useful to remember how spacetime has been modified in history.
The absolute Newton spacetime did not show any discrepancy between the proper time of a particle and the observed coordinate time. Newtons absolute spacetime and Galileis relative spacetime had been corrected by special relativity / Minkowski spacetime in order to take into account the absolute value of speed of light c. The pseudometric of Minkowski spacetime required the concept of velocity-dependent time dilation, including the Lorentz factor gamma.
Later, a second correction of the spacetime concept revealed to be necessary, in the form of curved spacetime representing gravity. This spacetime concept which includes gravitational time dilation is still today's concept. This concept is working very well, and no further corrections have been necessary up to now. That means that - according to our today's knowledge - beyond velocity and gravity there are no other causes of time dilation.
