Why do pupils dilate after death?

Pupils dilate after death because of the pupillary sphincter muscles associated with the iris (colored portion) of the eye. The pupil is just a hole in the eye! it is not a piece of tissue, skin or something tangible it is just a hole. The pupil appears as though it gets bigger or smaller because of the movement of the iris around it

The Iris is a muscle: It is the IRIS that is actually expanding or contracting in order to regulate the amount of light that enters the eye, when the iris gets bigger the pupil looks smaller when the iris gets smaller the pupil looks bigger.

When you pass away and your system shuts down your muscles experience rigor mortis where your muscles become stiff and locked in a certain position. The iris loses its ability to control movement and becomes locked in a loose retracted state, making the pupil appear to dilate.