Could be due to injury, could be a sinus infection alone, or a sinue infection linked to an upper respiratory infection. Mycoplasma is one of the big ones that affects the eye that way. It is extremely contagious if it is Mycoplasma. I would isolate the bird and put it far from any other birds.
Ok the eye would get a bobble so I treat her with antibiotics and cleared up but I have done this about 5 times for this bird and nobody ever seems to catch anything this time it's the worse I have seen so far that's all I can tell u. On a sidenote when we was picking birds we seen one hen that had it on both sides