Tearing of hemorrhoidal tissue is the primary cause of bleeding hemroids.  As with all hemorrhoids, blood vessels in the anal and rectal areas become stressed and strained and engorged with blood.  When this tissue tears, due to continued straining or difficulty passing stool, bleeding hemorrhoids are said to be present.

Only internal hemroids, that is the hemroids which occur in the anal canal and not exterior to it, can become bleeding hemorrhoids.  External hemorrhoids which occur near the opening of the anus do not become bleeding hemroids, and any bleeding that occurs in that area is usually due to an anal fissure.

It is also possible for thrombosed hemorrhoids to become bleeding hemorrhoids.  Thrombosis occurs when internal hemroids actually sag out of the anal opening.  Technically, however, they are still classified as internal hemorrhoids.  As mentioned previously, passing a hard stool can cause these hemroids to tear and bleed, but friction and rubbing can do so as well.