I was recently asked this question, So here’s the answer and I’ll warn you it may turn your stomach.
Wine is usually made from grapes, the juice is fermented with yeast which turns it into alcohol. Nothing fishy about that? Well… read on… During the fining of wines animal derived substances can be used. Commonly used: isinglass, (gelatine from the air bladders of freshwater fish) Second to that is egg albumen, ( a wine fined with this would be suitable for a Vegetarian) also used: milk/ milk casein, (OK for Vegetarians) gelatine, fish oil, blood, bone marrow and chitin, (component of the exoskeleton of crustaceans.) So this is why some wines are not suitable for Vegetarians and Vegans.
There are animal free alternatives for fining wines such as kaolin, plant casein and limestone. Unfortunately a lot of wine producers do not state what substance they have used, which is a shame. However, The Co-op label their own wines whether they are suitable for Vegetarians/ Vegans and also state the processing method, very helpful:) they get 5 stars from me!
Just because a product doesn’t actually have meat in it you can’t assume its Vegetarian/ Vegan.