The reasons for Sri Krishna rushing to attack Bhisma were that with his fierce warfare he was causing havoc in the Pandav army and more importantly Arjuna's repeated reluctancy to strike his grand father. Sri krishna's decision to attack Bhisma and to kill him brings us to the debate about his promise.
Many question the reason as to why would krishna break his promise which he made before the battle about not raising weapons and not actively fighting?
It's a relevant question and needs to be answered by remembering Sri Krshna's original promise in fact his eternal promise and his eternal occupation.
All the devatas along with mother Bhumi approached Brahma about the troubles caused by the demons disguised as rulers, Brahma in turn approached Sri Krishna who promised to appear in this world to set things right. Krishna appeared and started to establish dharma, by eliminating the demons or eliminating their rebellion.
Bhisma was not a demon in fact he was one of the most ardent devotee of Sri Krishna but Bhishma had chosen to fight on behalf of the Kaurvas, who were the last remaining burdens on earth.
Sri Krishna had a choice to break his eternal wow or his temporary vow.
Which vow would we ask Him to follow the real or the apparent?
The superficial people challenge him for breaking the apparent but the substantial people eulogise him for keeping his eternal promise.
What are we looking for the substance or the shadow?
What kind of questions are we raising the integral ones or the isolated ones? Therefore the very Bhisma seeing Sri Krishna strike at him dropped every thing, all his weapons and requested Sri Krishna to release him from his superficial commitment and allow him to follow the real commitment of being an instrument in the hands of God rather than in the hands of Duryodhana.
Therefore Bhishma who was eventually struck by Arjuna kept on remembering Sri Krishna rushing at him during the battle at the time of death and he wanted this event to stay forever in his very existence.
Because Bhisma knew Sri Krishna's eternal vow of liberating the bound and liberating the affected and tormented.
For this Sri Krishna the lord of all life chose to protect his original vow and not his temperory vow.