When a glass door slams shut, or swings back so fast it catches someone's heel, the closer is almost always the cause. Most commercial closers have separate adjustment valves for the main sweep, the final latch, and the backcheck that cushions the door when it is pushed open hard.
If the door used to close smoothly and has slowly gotten faster, the valves may simply need adjusting. Temperature matters too: closer fluid thins in summer and thickens in winter, so a door set in January can run fast by July.
If you see oil on the closer body or the floor below it, or the door ignores every adjustment, the seals have failed. At that point the closer cannot be tuned back into shape and needs to be replaced with a unit sized to the door's width and weight.
Leaving a slamming door alone is expensive. Every hard impact loosens the pivots and frame, stresses the glass, and wears out the latch. A short visit to adjust or replace the closer protects everything else on the door.