Soft Errors ("Single Event Upsets") are caused by radiation.
A Single Event Upset, basically means that a high energy particle (e.g. from a cosmic "particle shower") arrives in a semiconductor device (e.g. a DRAM part), and redistributes, or dumps enough charge there to change the logic state of information stored in some part of the device (i.e. flips a bit).
This radiation can come from within, or close to the device itself (e.g. from Lead which is used in device manufacture, or soldering), or from distant sources, such as cosmic radiation.
As of the time of writing, the vast majority of soft errors are believed to originate from cosmic radiation.
The use of BPSG (Boro-Phospho-Silicate-Glass) dielectric layers in semiconductors can make a device many times more vulnerable to cosmic radiation and most manufacturers have switched to other pasivation layers as a result.
There are a few references in this mailing list post:
Note that recent manufacturer datasheets which I have seen do however quote error rates approx (from memory) two orders of magnitude lower than the levels predicted by most references in the above mailing list posting.
If you have a large number of systems, then please share your experiences here and/or on the mailing list.