Why is zoom changing my plan from annual to monthly?

I've been on an annual plan for 5 years. Now, for the second year in a row, zoom has changed my plan from annual to monthly without my knowledge or request. Consequently I've been charged at the 16% higher monthly rate for the time it has taken for support to respond and get the annual plan reinstated. It's proving exasperatingly difficult to get the incorrect charge reimbursed.

Why is zoom changing my plan like this and what can I do to stop it happening? I just want to stay on the annual plan. Thank you.