What does it mean the the next annuity payment will be fully taxable?

If this a payment to you from your annuity then the total amount of the payment being made to you is from the interest you made during the growth of the annuity. Since the interest grew tax-deferred you must pay the taxes owed on that portion when it is removed from the product. It seems that the company is using the LIFO method of distribution which is Last In First Out. This means that any interest added to the product will be paid out first in most cases whereas taxes will be do on that money since you have not already paid taxes on this growth.