The Fallen are users who have witnessed too much to handle. Their mind goes into a blank-like state and leaves them confused and shaken. These people typically take a long time to recover and there is a risk of anyone in this category of going to the opposite side they fight for.
If a Fallen user does not recover, they will try to break their ties with anyone from the Wiki and will not recover in the real world either. If so, it'll be on their own time, but it's still there.