Like with other conspiracy theories, I suspect it's rooted in the same sentiments...
The underlying sentiment being, that for a lot of folks, there's something discomforting about the the reality that bad things can randomly happen, so there's something comforting about feeling like they have an explanation (even if it's not a good one) for many folks because if there's "an answer" than that means there's something they can do to prevent it.
Just as an example, take for instance the conspiracy theory about GMOs causing cancer.
The real answer of "many cancers are seemingly random and pop up in people for reasons we don't fully understand yet" is scary for a lot of people and leaves them feeling powerless over the situation. Because that means they could try to do everything right, and wake up tomorrow with an pain in their abdomen that ends up being cancer.
However, the conspiracy answer of "It's the GMO foods that are causing the cancers" provides them with at least some sense of comfort because even though the notion indicates something nefarious, it still gives them a sense of control over the situation and the feeling of "Since I know what's causing it, there's something I can do about it, I can buy non-GMO foods to protect myself"