That's a good question
I'll give you the "short version" of my understanding. I'll provide you will some great links as well.
Impacted crop is usually when a bird consumes foreign materials (like barbie doll shoes

), long grasses/hay/bedding and can't process all of it. I would suspect they are either overzealous in their eating and/or especially with grasses possibly don't have access to proper grit. I am in the camp that commercially purchased grit is to be made available. Yes, I do know that birds can most of the time find sufficient grit in the soil, but why not place a small container for them somewhere. Grit is relatively cheap and even a small bag will last for several years depending on how many chickens you have.
If you are interested, here's a video of removal of long grasses/matter from a crop that is impacted. I always find it amazing how much stuff they pull out. Not too graphic, but there is a smidge of eeewww

CROP SURGERY VIDEO
An impacted crop can become sour as well - things are not moving and bacteria/yeast begins to form (ferment).
Sour crop, slow crop, doughy crop - the crop is sluggish. A slow or sour crop,
can be a
symptom of underlying conditions like worms, cocci and reproductive/internal laying disorders. Basically there is inflammation and/or blockage somewhere else in the body that is delaying the processing/emptying of the crop contents - the contents then turn sour. There can be other causes of sour or slow crop as well which are in the article linked below.
@PD-Riverman After you do some reading on crop issues - I would be interested to hear what you are doing
correctly that you have not had an problems. It may be your feed, lack of excess treats(?), preventative maintenance (deworming), not keeping "old birds" - but I do help people with young ones as well.
@TwoCrows has a great article that explains crop issues much better than I can and lists some of the underlying causes and treatment options.
https://www.backyardchickens.com/ar...d-sour-crops-prevention-and-treatments.67194/