When a Fade cue targets a Group cue, QLab attempts to apply the result of that fade to all the child cues within the Group. Because the starting state of each child cue may be different, Fades which target Groups can only be relative fades.
This is a problem when your goal is to fade in several Video cues from 0% opacity to, say, 100%, because 100% of 0% is 0%. What to do? The trick lies in starting your cues at 1%.