Below is an article that goes into a bit of detail. Basically, high energy particles and waves are the cause. The earth is constantly being bombarded by all manner of particles and cosmic rays (high energy x rays). Also, the ground and building materials contain minute traces of radioactive elements which spontaneously decay, releasing alpha particles and other radiation. If one of these encounters a transistor, they may have enough energy to change its bit state.

Interesting, the problem is getting worse as chips components get smaller. Smaller components use and hold less energy, which means they're more easily disturbed by incoming energy.

http://www.edn.com/article/CA454636.html

[ QUOTE ]
Trouble Spots
The four common sources of SEUs are low-energy alpha particles, high-energy cosmic particles, thermal neutrons, and poor system design (Table 1). Low-energy alpha particles are generated by the radioactive decay of trace uranium-238 and thorium-232 in quartz filler used in mold compounds, or from polonium-210 in lead bumps that...

[/ QUOTE ]