Structural symmetry seems to have brought about the change: all other wh- words are monomorphic (consisting of only one morpheme).
The underlying cause of drift may be entropy: the amount of disorder (differences in probabilities) inherent in all linguistic systems.
[2] Cyclic drift is the mechanism of long-term evolution that changes the functional characteristics of a language over time, such as the reversible drifts from SOV word order to SVO and from synthetic inflection to analytic observable as typological parameters in the syntax of language families and of areal groupings of languages open to investigation over long periods of time.
Drift in this sense is not language-specific but universal, a consensus achieved over two decades by universalists of the typological school as well as the generativist, notably by Greenberg (1960, 1963), Cowgill (1963), Wittmann (1969), Hodge (1970), Givón (1971), Lakoff (1972), Vennemann (1975) and Reighard (1978).
To the extent that a language's vocabulary is cast into the mould of a particular syntax and that the basic structure of the sentence is held together by functional items, with the lexical items filling in the blanks, syntactic change is no doubt what modifies most deeply the physiognomy of a particular language.