Why does nasal congestion during a cold always alternate between sides?

It's because your nose has an automatic switching function called the 'Nasal Cycle'! Even when you're perfectly healthy, your brain's autonomic nervous system sends commands every 2 to 6 hours to adjust the congestion of the 'spongy blood vessels' in both nasal cavities. The brain causes one side's nasal blood vessels to become engorged and swell, reducing airflow in that nostril (allowing it to rest, moisturize, and filter air); simultaneously, it constricts the blood vessels on the other side, allowing for clear airflow to handle the primary breathing task. This way, both nostrils quietly 'take turns working'. Normally, when you're healthy, this switching process is very subtle, and you barely notice it. However, when you have a cold or allergies causing inflammation, your nasal lining is already highly swollen. At this point, the nostril that's 'taking a break' gets completely blocked in an instant, making you clearly feel the nasal congestion alternating sides!