I'm not sure what caused the drop but signs point to a mass exodus away from the product from a frustrated fan base.
Their annual financial reports have shown a steady decrease in overall sales for years, indicating a shrinking fan base, that they have been compensating for with regular price rises and massive cost cutting that only served to disguise and accelerate the problem, and those measures seem to no longer be working. Just before the big sell off they released a financial report that showed, I think it was, an 11% drop in profits compared to the same time period last year.
But...GWs mismanagement is probably at least partly responsible for letting all the new competition they have flourish, including FFG, so huzzah.