It's motivated by the desire to sell a $5/mo plan. Simply put, $50 a year is not compelling enough for annual subscriptions when you have a monthly plan that ends up at $60 a year. If they had decided not to offer a monthly plan (or set the price higher), they would have kept their annual plan at $50.
They see monthly plans as a way to get to a larger market, which explains why that drove pricing decisions.
Why give up so much revenue when it seems like price isn't holding them back?