No, it is not circular reasoning to claim that time dilation occurs because of the curvature of spacetime. The concept of spacetime curvature arises from the theory of general relativity, which provides a framework for understanding how gravity affects the geometry of spacetime.
According to general relativity, mass and energy cause spacetime to curve, and objects move along curved paths in this curved spacetime. This curvature affects not only spatial dimensions but also the time dimension. The presence of mass or energy can cause time to pass at different rates depending on the strength of the gravitational field.
Time dilation is a consequence of this curved spacetime. When an object moves through a gravitational field or experiences acceleration, it encounters a curved region of spacetime. As a result, the passage of time for that object relative to an observer in a different gravitational field or at rest can appear to be dilated or slowed down.
So, it's not a circular reasoning because the concept of spacetime curvature is not dependent on time dilation. Rather, time dilation is a predicted consequence of the curvature of spacetime caused by mass and energy.