Dogs tend to like a little variety in their diet and have a sense that "greens" are good for them. Some dogs will often eat grass because they like the taste and feel of the grass. This is the case with dogs who actually take the time to chew and swallow the grass.
Please remember that if this happens more than occassionaly, you may wish to consult your vet as there may be underlying medical reasons behind this behaviour.
Also, with spring upon us, many people have been laying out seed and other chemicals on their lawn, so beware for your dog's sake.