On the yearly subscription, do you charge for the entire year upfront? If not, you are eliminating reason 2) above to go for monthly
How is the distribution of vintages of your user base? Depending on your growth (and churn) rate, the distribution of old (>1 year old) vs. new (<1 year) users might drive more yearly subscriptions: the slower you grow, the more old users you will have in your mix of paying users. Old users already know they like your product and are much more willing to take the commitment of the yearly subscription to get the financial savings.
...so that might explain why you have a majority of annual plans. It’s always a good idea to experiment with your prices and segment the market, it’s almost always a good idea to do it without disrupting (too much) your existing user base.