Pg.143 small rulebook "Seized Standards...for every enemy standard bearer that was removed due to the Last Stand rule, or was slain in close combat (remember this can only happen once the rest of his unit except the champion are dead)"
It only happens in close combat but it can be captured if the standard bearer actually dies, so the standard is captured from slaying them (but not from shooting them to death) although I have no idea how it is effected by crumbling.