That's blowout. It's normal, especially for young pullets or hens just starting the season. When the egg pushes through the vent, small capillaries break when the surrounding tissue stretches. As the bird matures the skin will stay permanently stretched and there will be little or no blood. It can mean that the pullet is laying eggs that are too large for her body, in which case egg binding may result. Watch your birds carefully, though they should be fine.