Arjuna is referred to as Dhananjaya in this verse because he helped his elder brother in fetching wealth when it was required by the king to make expenditures for different sacrifices. Similarly, Bhima is known as Vrkodara because he could eat as voraciously as he could perform herculean tasks, such as killing the demon Hidimba. So the particular types of conchshell blown by the different personalities on the side of the Pandavas, beginning with the Lord's, were all very encouraging to the fighting soldiers. On the other side there were no such credits, nor the presence of Lord Krsna, the supreme director, nor that of the goddess of fortune. So they were predestined to lose the battle--and that was the message announced by the sounds of the conchshells.
Bhagavad-gita chapter one verse 15 Purport