At usual cruising altitudes (35,000-40,000 feet), an aircraft operates with roughly 100 to 300 times the cosmic ray and solar particle flux we experience at ground level. The Earth’s atmosphere shields from this radiation, so commercial aviation loses a significant portion of protection.
During a solar flare, the sun ejects high-energy protons that travel at nearly the speed of light. When these particles (or the secondary neutrons created when they collide with atmospheric molecules) pass through a semiconductor, they can deposit enough electrical charge to flip a bit in memory or logic circuits (0 becomes a 1, or vice versa). This is a known phenomenon called the Single Event Upset (SEU).
The Airbus advisory traces the exact vulnerability to their ELAC B (Elevator Aileron Computer) hardware running software version L104, the upgrade from L103. Flight control computers process sensor inputs and compute control surface positions many times per second. When a bit flip corrupts a value mid-calculation, such as an elevator deflection command, the output can be wrong without any error-checks.