The cannon will shoot itself at times if you haven't completed enough shots before hand, say like in the lower play field. Depends on the song playing, and it's objectives on that.
The auto plunger needs adjusted. Mine did that. Take a look at a ball sitting in plunger area and how the kick fork hits it. Mine needed bent together a little bit and works great now.

Mine was just doing that today. It goes about half way forward and shoots. I lifted the play field and pressed all the connections together. The one by the cannon was loose. It seems to fix the issue. I think my shaker makes it loose. Not sure if it will work for no you, but easy to try.

Opto switch, #62 I think, needs to be replaced on the cannon misfire. There is another AC/DC thread with the same issue; it's pretty common and aggravating.

I had seen a post about a light that is shinning under the playfield at the cannon that is causing it to misfire... mine was doing it intermittently.... I move that light and haven’t had a problem since

For the plunger problem check out this thread...

pinside.com/pinball/forum/topic/acdc-auto-plunger-is-starting-to-get-weak