It could just be the water in the pipe draining out. Once the sprinkler valve is shut off there is still water in the line to the sprinkler head (whats in your picture). If the sprinkler and riser in your picture are at a low point in the line and below your sprinkler valve the rest of the water in the line will slowly bubble out of the sprinkler head after it is shut off. Since there is no more water pressure with the valve shut it can take a long time for the water to drain if there isn't much slope. How long after you run the sprinklers does it continue to leak?
Or the valve could have failed like PX4 mentioned.