I think this is because of this account of the Battle of Strasburg, where the sight of the baggage guards led the Allemanni to believe that the Roman's had reinforcements, causing the Allemanni to break-
'They gave way, and the Emperor shouted aloud copying the words of the son of
Telamon. He, you will remember, told the Greeks that there was no returning home
for them once their ships were destroyed, and Julian told these men that if they
were beaten, the cities would shut their gates against them, and no one would
give supplies: finally he added that, if they had made up their minds to flee,
they would have to kill him first, for while ever he was alive, he would not let
them do it, and he pointed out to them those barbarians who were fleeing before
their pursuers. Hearing this and seeing this, they were ashamed and encouraged,
and they turned and entered the fray once more. Their disgrace was erased and
everyone joined in the pursuit, so that even the guards in charge of the baggage
train on the crest of a hill were moved to take part in the action. They rushed
forward and, their advance being seen, they gave the enemy an exaggerated notion
of their numbers, so that none now were ready to stand his ground. In
consequence. the field was littered with the bodies of 8,000 dead, and the Rhine
was hidden by the corpses of those who, unable to swim, were drowned, while the
island int he river were full of he bodies of the slain, for the victors routed
out those who wre skulking ther in the woods.'
