Wine is made from grapes so why are some wines said to be not suitable for vegetarians?

It’s all to do with the fining agents that are used to clarify the wine. A clarifying or fining agent makes wine clear by removing proteins from the wine. The agents eventually settle out of the wine. Some fining agents are animal-based products, while others are earth-based. Common animal-based agents include egg whites, milk, gelatin, and isinglass. Gelatin is an animal protein derived from the skin and connective tissue of pigs and cows. Isinglass is prepared from the bladder of the sturgeon fish, while egg whites and milk can also be used.
The quantities used are miniscule: whites from 3 eggs can clarify 250 litre of wine Both the clarifying agents and the removed proteins coagulate on the bottom of the wine tank or barrel. They are then removed through either a settling process or a cellulose fibre filter and are not present in the finished product.
It depends on how strict a vegetarian (or vegan) is as to whether the fining agent is a concern.