Merris, you know, I have often wondered and still do if it has something to do with a fear of failure.
My son, a couple of weeks before he could have earned his way home for Christmas, snuck out the window with a couple of boys after bedtime, and they were caught in the yard smoking a cigarette. He was already anxious about coming home because he was apparently very tempted when in the company of his neighborhood friends when he was home for Thanksgiving.
It was almost like it was a relief to him to settle the issue of the visit home by doing something to ensure that the possibility was taken off the table. I've seen him do similar things so many times.
Your son smoking pot two days before his release reminded me of this.