From my discussion on Language Log: This variant (which seems to be widely distributed in the U.S. and, from testimony on ADS-L, goes back to the 19th century) is clearly based on the disyllabic pronunciation of sophomore, with both the vowel and the offset consonant of the first syllable reshaped so as yield a familiar English word, south, in place of the unfamiliar first syllable of the original. Maybe some people think the compass point has something to do with the second year of college, but I suspect that the motive for the reshaping is primarily the search for familiar elements, for some people quite possibly encouraged by the south of the equally opaque southpaw. (Larry Horn, who made this suggestion on ADS-L, noted that historically southpaw is compositional, but with an etymology that hardly anyone appreciates; for most people, it’s just a idiomatic compound. For the etymology, see southpaw in AHD4.)
In this posting, I suggest that there are two drives behind reshapings: to find familiar elements as much as possible, and to find meaning as much as possible. Classic eggcorns show both effects, “demi-eggcorns” like southmore (and beyond the pail and cow-tow and many others) only the first.