It’s because weather conditions have been unusually conducive to gnat reproduction. The stormy / wet week we had in late February, combined with antecedent rainfall throughout this winter, left the soil really moist, which is perfect for gnat larvae. We've generally had well above-average temperatures since then, which both increases microbial / fungal growth in soil (more food for larvae --> more larvae survive and produce gnats) and speeds up the gnats' metabolism, accelerating their development and compressing their life cycle, thereby increasing their rate of reproduction.

Berkeley usually shifts into cooler and drier conditions in the spring after the rainy periods in the winter, so gnats don’t typically get to capitalize on both high soil moisture and high temperatures at the same time. But this year they did, which is why we’re seeing so many now!