Since he isn’t co-ordinating with Stannis, why is Crowfood Umber performing a suicide mission outside Winterfell, to presumably negligible effect?

A. It’s winter, he’s an old man from the North, therefore witness me shiny and chrome on the road to Valhalla, etc. 

B. He’s coordinating with Whoresbane Umber inside the castle. 

C. Not so negligible. He killed Aenys Frey, an experienced and competent commander whose known for being cunning and cautious, leaving command of the Frey army in the hands of Hosteen Frey, an aggressive idiot who’s highly likely to fall for Stannis’ trap.