>>4295492It's reticulation like
>>4295497 said.
The actual cause is temperature shock, ie going from cold to hot too quick, like if you're developing at 20 deg but it's winter and you rinse immediately in tap water that's 5 deg.
Remember that your film is a layer of gelatin thinly coated onto a plastic base. When it's wet and warm, it swells up and softens, if you shock it with cold water, it shrinks all over and these micro cracks form.
