The best explanation I've read is that the car bottomed out, lost a significant amount of downforce from the cessation of airflow under the car a few hundred yards from Tamburello corner.
The Italian courts were debating when the steering collumn failed (before/during /after the crash) for well over a year.
It was the right front wheel and suspension wishbones that broke off when Senna collided with the wall that delivered the fatal blow.