Most likely, they were from the rice fields and made its way to your home starting off as eggs.
A female rice weevil can lay up to 300 eggs during her lifetime, so its pretty easy to see how this can happen. They can be killed by extreme cold (freezing) or heat, so it is assumed that the rice manufacturer had less than state-of-the-art quality control practices, if the eggs are hatching inside your rice. However, it is also reasonable to assume 'a few' slip by the screening process every once in a while. Once hatched, they can multiply if its long enough for them to have matured into adulthood.