Genuine questions, but just speculating on a what if scenario.
Given that the Airbus is a fly by wire, side stick controlled aircraft and probably climbing on autopilot at the time of the incident, what would be the effect of a full excursion rear or forward movement of one of the side stick controllers.
Could a pilot inadvertantly knock the sidestick control full travel? Very unlikely I know, but would the aircraft respond immediately to the sidestick input, dump the autopilot and move the elevators full travel or would the built in protections prevent that happening? My understanding is that the controllers remain in position and do not self centre, so it might take a couple of seconds to recentre the control. Would that be sufficient to damage the aircraft?
Given that the repaired tail section might be slightly weaker than on an undamaged aircraft, could a rapid control movement cause separation?
I work as a test pilot, and have flown flight tests on various Airbus aircraft (but not the A321, although its control laws are similar to models I have flown).
Assuming the FBW system was in Normal Law, as it would be if there were no previous failures, full aft stick movement would result in 2.5g (if at high speed), or an angle of attack short of the aerodynamic stall (if at low speed).
In Normal Law, full forward stick would give -1g, which while dangerous for anyone not wearing a seat belt, would not cause structural failure. If the stick was held forward, the pitch attitude would reach the lower limit allowed by the FBW system (-15 deg?), and the aircraft would accelerate to a speed somewhat faster than the maximum speed allowed in normal operations. The aircraft would have been designed to safely fly at this high speed with full forward stick, and it would have been done many times during flight testing.
There are additional non-relevant nuances in specific corners of the envelope, but the above is a reasonable top level description.
In summary, there is no risk to the structure from either full forward or full aft stick, no matter how sudden the movement, as long as the FBW is in Normal Law.
I won't speculate on the possible cause of the accident.