Yeah, some of you found the best answer. The others aren't wrong either
Their greatest fear is a pathetic loss. After all, orks CAN be demoralized and routed. I thik it's part of their pre-programmed, instinctual war-making ability; They were made to be living weapons after all.
They will merrily throw temselves at the enemy and die in droves, so long as it at least looks like they're achieving something with it, but when their deaths at the hands of the enemy truly gain nothing, they will flee;
Note however, if escape looks impossible, they WILL turn back around just as quickly, and charge into their own deaths - after all, they might just get a slim chance of achieving something, and they'll die anyway.
Of course the orks themselves don't even think about why do they do so. It just happens.
Very well described in Ciaphas Cain: Death or Glory, and several other works.
Unless they are madboyz, because if they're feeling particularly crazy, their greatest fear might be anything the dice lands on, such as a certain colour, small woodland critters, or noisy machines.
My edit after Your edit:
Admin friend! That "problematic word" was in the original description of this behaviour, in an old Madboy rule.
Edited by MetalMammoth, 29 August 2020 - 05:31 PM.
replaced a potentially problematic word