^ That can be it, everybody knows that wookies are huge 2 meter tall tedd. erm.. beings.
The stormies were not prepared for a counter attack and got overconfident when the Ewoks arrived as backup for the Rebels.
I think this is what was going on inside those white helmets:
Stormie 1: Hey look, Teddy Bears!
Stormie 2:Cuteeeeeeeee! <3
Stormie 3: Is this supposed to be some kind of ambush? They are just rudimentary life forms; destroy them!!
[Battle takes place, the firsts to die are Stormie 1 & 2, the Ewoks win!]
Stormie 5: Rudimentary life forms my @$$...
Stormie 3: Oh shut up
baby Ewok : PWND!!!!