Kitniyot

Question

The newest Yated Neeman has a big headline NOTICE: Quinoa Should Not be used on Pesach by M Plaut Even though many have used quinoa, a South American plant that produces grain-like fruits, on Pesach, it should not be used on Pesach and should be considered kitniyos. Full details will be published in the Pesach edition. How can they say that a fruit that people only recently started using is kitniyot? If no one regarded it as kitniyot last year, how can they determine so this year. Kitniyot is a minhag and if the minhag was not to include this fruit, how can rabbomin now tell us differently?