At this point in the series, Eren is extremely hectic and anxious about the betrayal of Reiner and Bertholdt, who he sees as his 2 older brothers. A discussion happened prior to the scene where the scouts determined possible identity of the Colossal and Armored titans whichbwere reiner and Bertholdt. After all of this, Reiner manages to confirm Erens suspicions by claiming out loud that they were infact the armored and colossal. However, eren attempting to convince himself otherwise decides to be oblivious and pass it on as a joke or result of the battle, however deep down he knows its true. He and the scouts suspicions are unfortunately and ultimately confirmed when reiner has a monologue about his time here, and ultimately the both of them transform after being attacked by Mikasa. Eren is in a state of complete shock and emotion, to where ot drives him to tears. These tears however are soon replaced by anger, as he reminisces about their past and now sees them as traitors and attacks them with upmost rage.