Because the air in your tires might NOT be at "normal" levels at 20deg. If you last checked/filled them when it was warmer, then they would have lost pressure as the temp. decreased. Check them again, at the lower temp, and with the tires cold, of course. I think you'll be surprised.
If, on the other hand, you're sure they are at the correct pressure @ the, lower temps, then maybe the sensors, like many mechanical/electronic things, just doesn't like the cold? A little bit of moisture inside the tire ould have frozen on/in the sensor in just the wrong way to temporarily disable it?
