There are a number of possibilities. We likely will never know what actually happened. A bit flip would be bad, but potentially fixable. If they can somehow force a reset. It could also be simple component failure, a bad capacitor, in the wrong place, and your computer goes haywire. Ditto for mechanical damage. A grain of dust, hitting the wrong point could cause a cascade of problems.
The backup systems are long dead. The fact they've managed to extend the mission life by 41 years is quite incredible. It was never expected to last this long.