I am late getting to this, but my 5 year old seems to strike out when she feels like she is totally out of control. I have read that kids with spd are always in the "fight or flight" mode. So if your son is getting overloaded sensory wise and then having something sudden happen (like being pushed, or shut out of line) it may be that he is just reacting in the only way he knows how!
When we are having a bad sensory day, busy, lots going on, and my daughter is at her wits end and melting down, I will see her strike out, hit or throw something at other people. Normally she will not do this, she is friendly and easy going. But when overloaded watch out!
Maybe if you can talk to the teachers about giving "sensory breaks" letting him sit down for a few quiet minutes away from the other kids after a busy morning or activity you can avoid having this happen?