Anyone do sunflowers in their garden?

That's odd. You say that you've had them for months, so they should be pretty sturdy. Are there any spots on the plant's stalk or leaves? If so, they may have some sort of disease. I've never heard of any disease that actually breaks the stalk in half, though. I assume, unless you see some black, brown or white spots on your sunflowers, that it was something done by a person. Do you have any troublemakers in the area who could have broken your flowers? Maybe a large dog jumped on them?