A220 pilot reported during the descent they dialed in the new altitude from ATC; then all of a sudden the aircraft started a steep angle of descent. They regained control of the aircraft back to normal descent and speed.

Narrative

We were at FL400 and given a descent to FL350. I dialed in FL350 in altitude select and we verified 350.I initiated a VS descent at 1300 fpm. Aircraft was in a stable descent at .79 manual spd. I started my brief for arrival and at some point in descent ATC gave us further descent to FL240. I dialed in 240 in altitude select and again we verified 240. I continued brief and all of a sudden the plane started a steep angle of descent. I was caught off guard and proceeded to click off A/T and mistakenly hit the TOGA button. I noticed thrust levers move forward and saw that I had activated GA. I proceeded to turn off A/T's and tried selecting VNAV to turn off and VS to shallow out descent. Neither button responded and airspeed went into overspeed. I then disconnected autopilot and AT's and arrested rate of descent with the side stick and pull throttles back. I arrested descent back to normal descent of approximately 1500 fpm and stabilized airspeed and turned automation back on by engaging AP; AT's; and selecting VS at approximately 1500 fpm. Airplane descended down to FL240 at normal decent rate from approximately FL290. After we leveled off at FL240 and got automation back on we discussed what took place and FO stated he saw a VS of 9700 fpm and I stated that the aircraft went into overspeed. I sent a message to dispatch of possible uncommanded descent and overspeed. Once aircraft was at FL240 in ALT we were given OZZIE at 250/12000. I selected VS at 1100 fpm and aircraft descended without further incident. We continued approach and landed. Once at Gate I entered a write up in the logbook of uncommanded descent of 9700fpm and an overspeed.