Okay, so I have an idea on how this happened! Twilight went to take a shower at a private bath house and left his crystal inside the room he, Four and Sky were sharing. He specifically leaves instructions that it's not to be touched as he hangs it on the back of a chair, and promises to come back in ten minutes.
Four years it will be unsupervised for ten minutes so now's the time to check it out.
He and Sky cannot resist temptation.
Twilight comes back and finds them like this. He decides not to tell anyone, 'till he can figure out how to break the curse.











