the OED seems to have taken the approach of using -ize for all words except those already well-established as -ise stemming from French through Latin:
In med.L. and the mod. langs. these have been formed also on L. or modern national names, and the use has been extended to the formation of verbs from L. adjs. or ns. This practice prob. began first in French; in mod.F. the suffix has become -iser, alike in words from Greek, as baptiser, évangéliser, organiser, and those formed after them from L., as civiliser, cicatriser, humaniser. Hence, some have used the spelling -ise in Eng., as in French, for all these words, and some prefer -ise in words formed in French or Eng. from L. elements, retaining -ize for those of Gr. composition. But the suffix itself, whatever the element to which it is added, is in its origin the Gr. [], L. -izare; and, as the pronunciation is also with z, there is no reason why in English the special French spelling should be followed, in opposition to that which is at once etymological and phonetic. In this Dictionary the termination is uniformly written -ize.