Mainly bleeding gums are caused by inadequate plaque removal from the teeth at the gum line. This can lead to a condition called gingivitis, or inflamed gums.   If plaque is not removed through regular brushing and dental appointments, it will harden into what is known as tartar. Ultimately, this will lead to increased bleeding and can lead into a more advanced form of gum and jawbone disease known as periodontitis.   Other causes of bleeding gums include:
Any bleeding disorder
Brushing too hard
Hormonal changes during pregnancy
Idiopathic thrombocytopenic purpura
Ill-fitting dentures
Improper flossing
Infection, which can be either tooth- or gum-related
Leukemia
Scurvy
Use of blood thinners
Vitamin K deficiency