Sounds gay, soppy and I will probably be called an idealist, but have you ever thought about taking them on their own, sitting them down and perhaps setting the scene with some munch, some beer before RAPING THEM.
Ahem, I mean sitting them down and asking them why they said that? Dont try and act defensive as they may do, ask them as though you are genuinley curious. If you ask them while they are together, you wont get anywhere. If you ask them on their own, they are either likely to act really defensive, admit and say why, or tell you it wasnt them but expain why the others dont.