I have a venous leg ulcer it,s a large one and it has started to bleed heavily, what will cause this?

If you truly: have a venous leg ulcer (chronic) then you need treatment at an advance wound center where proper diagnosis and treatment usually result in fairly prompt resolution. For the time being apply direct pressure to the wound with multiple gauze pads to control bleeding then replace with non adherent dressing until you can be seen.

Erosion: Erosion ,or spread, of the ulcer into a significant blood vessel. See a venous specialist and a wound care physician.

Bleeding vein ulcer: Venous ulcers are due to chronic venous insufficiency which leads to venous hypertension. The area around the ankle has the greatest pressure and the skin weakens and can then ulcerate. Blood vessels in that area can erode through the weakened skin and then bleed. If you have a venous ulcer then you need a full venous evaluation with a venous ultrasound. See a vein specialist.