The spoiler said "dangerous secret", so I'm going to say it is dangerous for her because she saw who really killed Diane because when she left the group on the day of their outing she came to GC and just happen to be in the park at the time when Diane was killed.
But then for some reason I have a feeling it has to do with Deacon since he hasn't been around since Diane's murder and a source of mine says Nick punches Deacon.
ETA: Maybe the punch is because Deacon tells him about the dumpster sex with Phyllis and Nick hits him because he thinks he's lieing.
Guess we'll have to wait and see. Hopefully it won't be drawn out forever.