It's definately not tied to:
Killing people
Dealing damage
It's definately tied to:
Surviving for a long part of the game
It might be tied to:
Holding one area for a long amount of time. (really hard speculation, I find it highly unlikely myself. Basing this on my old friendgy xbow campers getting valour all the time)
Overall I'm at a loss what else could grant valour, since easily recognizable/scriptable variables seem few for situations like this in m&b.
I often don't get valour if I charge in at the beginning of a round and kill more than 4 people, yet I often get it if I'm the last guy to die. Dealing lots damage - such as hitting multiple people with my gs once or twice also has always failed to grant me valour, even in extremes when I run in - land about 7+ or so hits and die without kills.
I think everyone has noticed that the most cowardly, ungallant and un-valorous get it far more than those who should.
Valour atm awards people who try to ignore the first initial clash and hang back. I hate it as it is now, I admit I tend to play recklessly and often die in midgame or so and tend to not get valour even when I land 6+ kills.