Clearly something about modern life is causing GERD, but it's not clear exactly what it is. As countries "modernize", it seems that an increase in GERD soon follows.
Sometimes I think it may be due to the rise in the consumption of meat. I don't think it's really natural to eat meat on a daily basis, and historically it was more of a treat that people ate only occasionally. Most people eat a lot more sweets and salt now too since everything is loaded with high fructose corn syrup and salt to make bad food taste good.
Another thing I wonder about is the proliferation of pesticides in the food chain. As we spray this stuff on all our produce, it slowly makes it's way into our bodies in small quantities. We're not that different from the insects we are trying to kill, so if it's bad for the bugs, it's probably bad for us too.