Some small farmers want to protect their crops and lands. Think "small business trying to stop the competition." This can be billed as a David-and-Goliath kind of struggle if you like, but the GMOs don't make the neighbors' farms any bigger or smaller. There's the risk of contamination, but those heritage farmers' fields are next to fields growing various hybrids anyway, so "heritage farmers'" stock gradually shifts. But since they're sort of all-natural and anti-GMO branded, some genetic shifts are far worse than others.
Some are people who just plain fear science and change. They don't like microwaves, they don't trust cell-phones, they hate plastics and really, really are terrified of ornamental kale. 
They're really into fear and what they don't understand. Since they don't understand large portions of life, they have no fear of a hell in the afterlife because it just can't get worse.
Another group worship at the altar of the Precautionary Principle. If it's not proven safe, it's dangerous. Note: Salt has not been proven safe. For grandfathered-in substances, traditional ones, they're tolerant. For innovations it must be proven not just safe beyond a reasonable doubt, but safe beyond all doubt. They like strict standards for approval, so most things that are currently in use would fail the test. When something they don't like passes all the tests, they doubt the methodology of the tests and call for research to improve the stringency of the tests.
Some anti-GMO folk are just anti-corporatist or anti-large-company. They're not opposed to people having large amounts of power; it's a trust issue, they want people like themselves (and only like themselves) to have power. These are the ones that have the strongest urge to appeal to authority.
Politicians can be in any group. Often they're in the fourth group--they have power and like to wield it. If they one out of things to control, then all the power's with the bureaucrats because all the "good" laws are written. They play off the economic fears or desire for empowerment of the first, the incompetence of the second, find the precautionary principle to be fertile grounds for breeding and developing new laws and entire new genera of laws, and delight when they are the people that others find to be like "themselves" and want to give power to.
It's had a hard go in America, where we tended to be individualistic, had a traditionally large group that looked to science and engineering for progress and found the precautionary principle to be a hindrance to a lot of things, and esp. dislike giving away too much of what they (apparently foolishly) deemed their own power to others.
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