When I see this, I always assume slugs, bunnies, or other critters. But, in perusing the research, it seems there are also other causes. Too cold, too wet and the answer below from the Texas Extension Service:
Occasionally green beans germinate and come up but only have two leaves or maybe none at all. What is wrong?
A. This condition is termed snake head or bald head and is generally caused by planting cracked or damaged seed. Occasionally the beans literally pull their heads off when forced to germinate and come through heavy or crusted soil. Planting high-quality seed and maintaining the soil in a relatively moist and friable condition will help eliminate this problem.
Thank you, Ann for your prompt and considered reply. It doesn't always happen. Now that I know what the problem is called, I did some research myself and came up with the same information. I did use fresh and high quality seed with no evidence of injury. The cause was probably crusting. I know germinating beans are tough so wasn't concerned about the surface condition of the soil. Also the temps may have been a bit cool as I planted just before the weather turned chilly and rainy which lowered the soil temp and crusted the surface. Thanks again. I've been gardening 42 years and continue to learn. Peggy
You are welcome. The only way I learn new things is for someone to ask a question that I need to research! Hopefully your next planting will come up looking great.