When you’re building a business based on recurring revenue, what you want is predictability. The “recurring” part is a pretty strong measure of that, but getting more of that recurring payment upfront reduces churn and improves cashflow, letting you spend more money now to acquire customers faster. But turns out there are some methods of getting customers to switch to an annual plan that are much more efficient than others. Context matters!
I've seen similar increases using in-app messages. The filter we apply is 2 successful stripe charges. Seems to give them enough time to have worked out if the service solves their problems and then offers a way to save 40% by switching.