The crop is a storage for food, basically. The food is held there as food passes thru the GI Tract. The gizzard grinds up anything that is not water soluble. So if the gizzard is packed full of grass and such things and takes too long to move, the food in the crop can sour. There are healthy bacteria in the crop to keep the food from going bad. But when yeast and fungus start to grow in the crop from food sitting too long, it "goes sour" as they say. If a bird has a full crop first thing in the morning on the roost bar, something is wrong. A crop should empty out completely after 1 to 4 hours. Chickens will stuff their crops before roosting time to give them as much food as possible to get them thru the night. The crop should be completely empty by morning.
So, you want a low ph crop. So raw, unfiltered apple cider vinegar to the water, 2 tblsp for maintenance, 3 for a sick bird, will keep the crops healthy. I add it once a week for good health. If a bird is on antibiotics, this is a must for the water during and a week after the meds.
The use of probiotics in the water or feed is good also to keep the healthy bacteria growing.
Grass is the number one cause of sour crop. It takes a long time to grind down, and if the chickens eat it all day, the system can not keep up with the grass grinding and the emptying of the crop. Hence, sour crop.
It is difficult to treat if it has been going on a long time and you do not notice it. Keep an eye on the crops by occasionally feeling them first thing in the morning.