I I simply don't understand why anybody who is already AG would covet the job of CJ.
Anyone who dreams of being remembered will choose CJ over AG. An AG is just another prosecutor, after all. A very fancy one, to be sure, given that he/she also "advises" the government---but, still, just another one. On the other hand, a CJ with sufficiently strong will and skills can make history in both the creation and enforcement of the law.
In a country with a reasonably-functioning judicial system, the average citizen is unlikely to remember any past AG and will have only a foggy idea of the present one; that would be very different when it comes to the CJ. On the other hand, in backward places, the AG tends to be better known or remembered because he/she tends to be the sharp end of the anti-law, oppressive stick.
So, here we are in Kenya. Amicus Kassin Consigliere could go for CJ for any number of reasons. For example:
(a) He has genuinely repented and wishes to leave behind a legal legacy that his children (illegitimate & otherwise), friends, and compatriots will admire; or
(b) his masters, be they cousins or friends, consider that he would be of better service in that position, much as in the world of spooks "sleepers" should always be upgraded until they reach the "peak"; or
(c) having noted that today's CJ in Kenya is better protected from the whims of politicians and also having noted Henry Kissinger's remark that raw power does amazing things for the libido, he simply is mindful of Mrs AG.