Why is it when a character nimbly dodges attacks the aggressor will call it "running away"?
Many people will accuse someone of running away in a fight if they do a lot to evade being hit by their opponent. Generally in a fight one can tell wether an individual is actually "running away" or just trying to avoid getting hit. Someone who does not actually want to be in the fight or is scared will generally keep running away with out making an attempt to actually fight back. 

In the case of One Piece we see a lot of characters dodge and evade but that reason is to just avoid damage. Luffy for example will dodge a lot of attacks but he will make every attempt to inflict as much damage as he can on his opponent. He will even fight back when he does no damage at all. (i.e. Magellan) 

Dodging and evading is not considered cowardly most characters will just call it so becasue they are fustrated that they have missed their attacks and wish to goad their opponent so that they may stop all of their evasive maneuvers.