What I have heard about chakra vyooham.It is supposed to be known as Padma Vyooham in which Abhimanyu died,,i think
Suicide warriors belonging to kauravas side challenge arjuna and they take him away by taunting him.Because his ego was punctured by these guys,arjuna goes after them inspite of Krishna's advice.Meanwhile, as there is no one to enter Padma Vyooham,Bhima requests Abhimanyu to enter as abhimanyu claims knowledge of entry.Bhima assures abhimanyu that he & other pandavas will also help him in his return.Abhimanyu enters the vyooham with Duryodhana being the central warrior in the vyooham.Jayadratha had got a boon from Lord Shiva that he can defeat all pandavas except arjuna for a day.He uses this boon and wards off Yudishtra,Bhima,Nakula and sahadeva from following abhimanyu into the vyooham and they are unable to help him.
I think it was one of duryodhana;s son who finally kills abhimanyu with the blow in the back.By that time, he is very tired as all his weapons are broken.Karna breaks abhimanyu's bow by shooting from behind as per Dronacharya's advide.All these actions are adharmic.