Looking over the contribs for Mr. Sjc (T-C-L-K-R-D)
, it looks like he's taken lengthy breaks before, most recently from Oct. 30, 2008 thru April 9, 2009, following what appears to have been a talk-page blanking on Aug. 8, 2008. However, I don't see anything that would have precipitated that reaction... In fact, the static he got over the Authonomy (T-H-L-K-D)
article (which is hardly something they should be deleting) seems to have been a fairly isolated incident.
My guess is he just outgrew the place, and reached the point where he just needed a decent excuse to quit. Like Jimbo says, people will stop bothering when it stops being fun, and it's hard to imagine people having much fun on Wikipedia. (That's always been the case, but now it's a little more obvious than it was.)