Bleeding in the human body can be caused both by mechanical factors (trauma) and chemical. The last include many medications, as well as disturbances in coagulation factors. If this is the real cause, the bleeding can appear on any part of the body. If there is little quantity of blood that originates from the rear pharynx wall blood vessels, then the mucosa of the larynx might somehow be affected. An inflammation, both acute and chronic in any part of the upper respiratory tract (nose, mouth, throat, larynx) can make the rear pharynx wall mucosa more susceptible. The mucosa will be more irritated and can even be damaged, resulting in small bleeding. More intensive bleeding can be due to pharyngeal varices, or hemangioma (usually a benign tumor originating from blood vessels). You might want to visit an Ear, Nose, Throat specialist to determine the real cause.