Algae are tough. There are thousands of species of algae and many can thrive with a handful of nutrients and a small amount of light, even in the freezing temperatures of your pond.
The key ingredients for algae to grow are food and light.
Food comes from decomposing organics in the pond which produce phosphorous and nitrogen that the algae feed on. In warmer times, natural bacteria break down these nutrients thus cutting the food supply for the algae. In the cooler months of winter, the same bacteria go dormant and the algae can often thrive. Winter bacteria additives can be one method of regulating this winter bloom.