I think the relaxed and sleepy is a major factor. My biggest fear was on the way back from our away games and matches, from very early puberty all the way through college, when I was tired and maybe dozing I would always get a major boner. I wasn't the only one, but there was always a few guys who either stayed awake to prevent it or could prevent it from showing and they watched the rest of us to pop wood then point it out to everyone on the bus.
wrestling was worse since it was very common for us to freeball under our sweats on the ride back. I see guys, even college guys, now who go to extremes to make sure their lap is cover before they fall asleep. Bringing blankets, keeping their backpacks in their lap, pulling the bottom of their hoodie down over their knees, etc.
As frequent as they were in the height of puberty, I would say they were not just random. I soon learned to think back and could almost always find a trigger. A place, who was around, or what I had been slightly daydreaming about. Staying focused on non arousing things all day back at that age was quiet a chore, impossible for most, including myself.
