Why do my gums bleed?: Dr Ian Kelly, Dentist
If the gums are irritated you will have an inflammatory reaction, which basically increases blood flow. My favourite analogy there is the over-inflated balloon - if you have plaque present there, the gums will react. Blood flow will increase.
If you don't remove the plaque they will continue to react. The blood vessels dilate and dilate and dilate, and in the end they become weak, they rupture and you'll have bleeding.