My guess was based on knowing that his parents were likely conservative (owing to the signs in the yard). In my experience conservative parents are often stricter than more liberal leaning parents. If that was the case here then knowing he was bullied and denied being on the rifle team, I can imagine that things weren't exactly chipper for him in the family home either.
My guess was, of course, just speculation. However, I would urge whoever is investigating his actions to look into the family situation to get a better picture of what might have motivated the young man's actions.
I get the feeling they didn't think this through thoroughly.