Summary: Sometimes the safest course of action is inaction.
The Doctor would sooner hide than fight. Fighting means violence, and violence all too often leads to injury, destruction, death… Innocent lives lost for reasons that seldom make good sense. Fighting is not his way, or so he repeatedly tells himself, because he’d rather not remember the times he’s turned his hand to violence. The War Doctor is a facet of himself he’d prefer not to acknowledge.
In most cases, hiding gives everyone a better chance of survival, it’s the safest course of action, but when it doesn’t work… That’s when the Oncoming Storm makes the universe tremble in fear.