Why is "Syndra" a forbidden name?

I tried getting this name since Frostmourne S1, but it's never avaliable. There's no Syndra in the alliance neither in the horde when trying to add friend, theres no Syndra in the armory, yet when trying to create a character with this name it just says "This name is unavaliable."

I cant see any reason why this name would be offensive or anything... so why is it always unavaliable?

The list of reserved names is made by Blizzard and they have not provided a reason for each of them. In this case, the list of reserved names, included in the WoW client, contains an entry that forbids using any name that starts with "Synd". So it's something more general and not targeted at that specific name.