I agree with you that normally autopilots should not do that. If something unusual happens and the autopilot does not know how to deal with it, like power kept on, it will disconnect.
Either due to excessive pitch or overspeed. The question is when it disconnected. But the trim on the Boeing is aparently very fast in trimming when on autopilot. I am not a Boeing guy, but somebody here said it before. Even if the autopilot disconnected early enough, the flight directors would still guide the pilots to nose down. Similar thing happened with many goaround accidents. I still think the autopilot disconnected on them. Could anybody who has access to the Boeing sim try to set a low goaround altitude, or try to do a goaround from intermediate altitude, keep autopilot on, set toga and "sleep through it" with no action? I wonder what it will do. Airbus will pitch down and accelerate and you have to be quick in retracting flaps. I still have to find the accident where airbus test pilots got killed this way. At least that's what I remember hearing in 2008. And of course, violent, or brisk is a question of how we define the words. But at 2am, even brisk can feel violent.