Full disclosure -- I have no answer to the above question, only observation and conjecture.
Last year during November and December I faced some rather dire circumstances with a cancer diagnosis. I had surgery in November and was consequently in and out of emergency rooms and hospital stays through November and December. My dog was usually in the car when I went into an emergency room and disappeared into a hospital for 5 to 7 days.
In December, Nestle started having pretty severe separation anxiety in the car. He would get into the front, into the wheel wells, and chew up whatever he could reach, including the rubber pads on the pedals, a water bottle in the car door holder, the knobs on the radio, the plastic things to hold the floormat in place, the cover of the 12-volt plug, and a couple of times part of the dashboard.
I wasn't feeling up to doing much until March, but then I started working on a long desensitization program to get Nestle comfortable with being left in the car again. It was a LONG road, but by September we were starting to do pretty well, and the last couple of months we were finally able to leave him in the car with him staying in the back and not drooling all over himself.
Then we had a cold snap and the first dusting of snow this last week. (It was quite cold and sometimes snowy last year at this time.) And all of a sudden I came back to the car to find Nestle trembling on the floor under the steering well, the top chewed off my water bottle. Yesterday I was IN the car with him while my roommate went to a doctor's appointment, and he was leaning on me and trembling.
As far as I know, nothing bad has happened while I was away from the car. So I am forced to wonder, has the weather triggered this? Is he "flashing back" to last year because the weather is now the same? I can't think of any other possible reason, but I'm not sure if this is a capability that has ever been established in canines.
In any case, I am now starting again on desensitization. I guess this time it's to the season as much as the car. Sigh.