At 0:28 the top of the page in large letters say "The Identification of Unknowns." Why would Chen be reading that book? Because he wants to figure out his powers! Unknown = Powers. Perhaps this means he does not know the unknown lengths of his abilities and is finding a way to control them. Notice how in all the other teasers they are running away, afraid, or have a sorrowful look on their face. EXO doesn't how to use control their powers anymore, so now they are looking for an answer!