It has to do with what type of agents are used to "fine" the wine. "Fining" the wine is what they call it when they remove the cloudiness from the wine to make it more clear. If they use activated charcoal or bentonite then the wine can still be considered vegan. However if they use casein ( milk protein), albumin (egg whites) , gelatin (animal protein), or isinglass (fish bladder protein) it is not considered vegan.
Tammie is right! There's a whole lot of disgusting (I mean, seriously?? Fish bladder???) shit in wine. It can also be dangerous for people who are allergic since the companies doesn't have to state anywhere that it contains egg or whatever.