In 2015 it felt like sky was the limit for Red Velvet but then the entire game changed when Twice became NGG. In 2016 RV came back with OOTN and Russian Roulette. It was also their the airing of their first reality "Level Up", Joy's first acting role (which got record low ratings), and also Irene's first web drama. The public wasn't as interested in The Velvet, and RR album didn't get as much hype from fans as The Red or later RV albums. It also felt like RV was on "decline" that year even though they were still one of the top (probably 2nd) after Twice.
Then 2017 came and Red Flavor took RV's popularity to another level, so their history in 2016 was forgotten. 2020 would also be considered another "lost year" but because corona happened it was the case for everyone at different degrees.