Rabbi Gil Student notes that Rabbi Yisroel Belsky, a haredi gadol (and, I must add, a nice man who has stayed out of most of the recent spate of bans) has banned Quinoa and Amaranth for Passover because he considers these non-grains non-legumes to be kitniyot. Of course, this is absolutely not true scientifically.
A brief history lesson. Why was corn banned? After all, it is not kitniyot.
Corn was banned because the word for "staple grain" in many European languages is "korn." Indeed, the corn we eat is really maize. It was called "korn" and later "corn" by Europeans because it was the staple food of Native Americans. Because maize had the same name, "korn," as "staple grain", it was banned.
A story is told about Rav Moshe Feinstein. A man came to him with a strong case to ban rhubarb on Passover. Rav Moshe heard the man out and then replied, "Wasn't corn enough for you?"
Rabbi Belsky seems to hold that Quinoa and Amaranth – commonly but mistakenly called "grains" – should be banned for the same reason corn was banned. Even though Quinoa and Amaranth are not the "staple grains" of modern America, apparently Rabbi Belsky believes people will mistakenly come to eat wheat berries and the like on Passover if the eating of Quinoa and Amaranth becomes widespread.
Rabbi Student also notes that Rabbi Hershal Schachter permits Quinoa and Amaranth. So do the CRC and the Star-K. So those of you who want or need to eat Quinoa and Amaranth have what to rely on. But if you do, you need to check each grain to makes sure no wheat or other true grain has become mixed in.