Ah OP I could have written this about ADHD DS. For what it's worth, he's now 15 and out the other side, but (and I say this as his mother who would do anything for him) he was a rude, insufferable arsehole from 12-14. Nothing triggered this. Nothing stopped it. It was a horrible phase that he simply appeared to go through as part of puberty.
Was it just his personality? Was it exacerbated with SEN? Don't know.
We had blazing rows, he was horrendous. We had to "fight" back because if we didn't, he got worse. Honestly those two years were fighting and not a lot else. He would be indignant at the mere suggestion he couldn't speak to people that way, and we all "needed therapy/were shit parents/morons/idiots who shouldn't have had children/to blame for everything". Got to the point he would smash things in defiance.
He's literally just stopped of his own accord. I wish I could tell you what to do, we tried everything. Kindness, long talks, discipline, no discipline, shouting, sanctions, rewards, routine, no routine, structure, no structure. His consultant had no more ideas and essentially said buckle up and wait for it to pass. And it did.
I wish I knew then, how different it would become, because at the time, I can remember thinking, I can't do this anymore, he's going to end up either beaten up or in prison if he launches into the world behaving like this as an adult.
Sending you strength, you're not alone. It may sort itself out quicker for you xx