It was rather common to assassinate the emperors, and it was not rare for the subordinates to kill the emperors and propel their own masters to the throne.
It was mostly pretentious to claim that the emperor's robe was forced on somebody, though it happened occasionally in history.
It was certainly the case that the Grand Scholar Zhang didn't know about this assassination, neither did the Emperor; but many people knew about it in advance.
The guards in the Royal Palace were all appointed by the Grand Scholar. Though they didn't receive the direct order, they knew where the assassins in the water cart came from and kept silent. Those eunuchs who had heard the information were trembling in their quilts and didn't dare look out through the window. The whole Royal Palace was shrouded in an eerie quiet.