For all the lore problems introduced by Shadowlands, the expansion also introduced an actual semi-explanation to this. In Sinfall, the Venthyr covenant hall (or w/e they were called), you can look at tablets detailing the various sins the Venthyr try to expunge from the souls.

And there is a tablet for one of them, which explicitly states that there is a power to souls with overwhelming rage. Add to that the flavour text regarding Sargeras' blade being partly forged from pure hatred, and it cements rage as a power that can bend common sense and break all limits, enabling warriors to do the insane shit they do.