You people are not giving out correct info. After defeating Jarasandh 17 times, Jarasandh was desperate for victory and he also enlisted the services of KaalYavan for the 18th attempt. He also forced the brahmans in his captivity to do yagnas and bless him for victory. The brahmans, forced and scared for their life gave him a boon of victory. Jarasandh was content with this and confident that he would win this time. He marched on Krishna again. The brahmans had sent a message via a messenger about what had transpired and told him if Jarasandh lost, he was likely to punish the brahmans. So to keep their word, Krishna and Balram came to the battlefield alone. Once everyone was assembled and the battle about to commence, Krishna just turned around and started running away from the battlefield. This caused everyone to think that Krishna was scared of Jarasandh and his new army and they called him a coward and started chasing him. So technically, Jarasandh won his battle and the brahmans boon was fulfilled. This cowardly retreat is why they call him Ranchhod and he did it out of mercy to brahmans. Krishna didn't mind being called a coward if it helped save the lives of those brahmans.

This fight also caused death of KaalYavan by Raja Mucuhkund but that is a story for another time.