SEUs can cause problems with electronics in space. That's why they're built to be radiation hardened.

Single event upsets (SEUs) occur when a single ionising radiation event produces a burst of hole-electron pairs in a digital microelectronic circuit that is large enough to cause the circuit to change state. In space, these unplanned changes of state usually result from the direct ionisation of single heavy ions originating outside the spacecraft, or by the products of nuclear reactions initiated by particles that originated outside the spacecraft. The Cosmic Ray Effects on MicroElectronics (CREME) model suite (Adams, 1986) was developed by the Naval Research Laboratory (NRL) to assess the SEU rates in the radiation environments that can be expected during space missions.

I'm not saying that Spirit's problems were caused by a SEU. All I'm saying is that SEUs can cause random bit flips in computer systems, often with unpredictable results. If you flip a bit at random in memory, you may corrupt part of the operating software or data stored at that memory location.