Hi Scorpio,
Purvi has explained why Arjun attacked Bhism. Let me know if you need any more detail.
Arjun was killed by his son Babhruvahan. But it did not happen in Mahabharat war. In fact, Babhruvahan did not even participate in Mahabharat war.
Pandavas won the war and Yudhisthir became king. Yudhisthir did ashwamedh yajna. In ashwamedh yajna, a horse was let loose after doing certain rituals. Army went behind the horse. If the horse passed through a kingdom and the king of that kingdom did not stop the horse, it meant that the king treated the doer of ashwamedh yajna (Yudhisthir in this case) as superior to himself. If that king stopped the horse, then he had to fight with the army, which was sent behind the horse.
Arjun was heading the army. Many kings accepted Yudhisthir as superior without any fight. Some stopped the horse and Arjun defeated them.
The horse moved to where Babhruvahan was ruling. Babhruvahan's mother was Chitrangada - who was a wife of Arjun. When Babhruvahan heard that his father had come, then he became happy. He went to welcome his father. Arjun got angry. He thought that Babhruvahan was a coward. He could not tolerate that the son of such a great warrior like Arjun could be a coward. He wanted Babhruvahan to fight with him. Out of anger, he criticized Babhruvahan a lot.
Ulupi heard all this. Ulupi was also a wife of Arjun. Babhruvahan gave as much respect to his step mother Ulupi as he gave to his own mother Chitrangada. Ulupi remembered goddess Ganga's curse. He provoked Babhruvahan to fight. Babhruvahan got provoked and a terrible fight happened between Arjun and Babhruvahan. In that Babhruvahan fainted and Arjun got killed.
After some time, Babhurvahan recovered and was crying because he had killed his own father. Chitrangada came there and was shocked to know that Ulupi had provoked Babhruvahan to fight with Arjun. Both Chitrangada and Babhruvahan decided to give up their lives because of shock. But Ulupi stopped them.
Ulupi had a gem, which could bring dead to life. She gave the gem to Babhurvahan and asked him to put it on Arjun's chest. Babhruvahan did the same and Arjun became alive.
Everybody was happy. Then, Ulupi explained about Ganga's curse. Actually, the curse was given by Vasus and by Ganga together. Ulupi explained all this to everybody present there.
I have used the word Vasus here. Vasus are group of gods, who take care of some natural phenomena. Bhism himself was also a Vasu, who was born on Earth. When Arjun killed Bhism by keeping Shikhandi in front, then other Vasus and goddess Ganga gave the curse to Arjun (which I mentioned in previous post).