The bacterial plaque is a primary cause for bleeding gums. The plaque contains bacteria and they release toxins which are irritant to the gums. The gums swell up, fill up with blood, and become fragile - that's why they bleed when we 'poke' them.
It is like a splinter underneath your skin - your body knows it is a foreign body and draws all the blood with white blood cells to the area, to try to eliminate the irritant; but you need to remove it yourself to help your skin become normal again. The same rule applies to the gums - you need to remove mechanically by brushing and flossing.