Well one way of looking at it is as follows.
Imagine that we exist within a 4 dimensional environment, a Space-Time environment. Now imagine that we have an object that extends across space and that this object is at rest in Space. However, this object is still in motion. It is still in motion across one of those 4 dimensions. It is in motion across the dimension that is known as the dimension of time.
Now imagine that the magnitude of this motion is equivalent to the speed of which light moves across space (c), and that this specific (c)onstant motion applies to all objects.
No matter what direction any of the objects travel within Space-Time, this specific magnitude of motion is maintained. If a bus turns at a corner, the bus has changed its direction of travel, and thus in turn the bus has rotated. Thus, if any object, as it moves across Space-Time, changes its direction of travel, here too, rotation occurs.
Now, if you take all of this into account and analyze the outcome of such circumstances, you encounter Length contraction, Time dilation, Lorentz Transformation equations, the Velocity addition equation, and the relativity of Simultaneity.
You also find that under such circumstances you will measure the speed of light as c, and do so no matter what frame of reference the light is being measured from, nor does it matter which direction the light is traveling toward.