The canonical understanding of disyllabification is the evolution from monosyllabe to disyllabe but not odd-number polysyllable to even-number polysyllable. The morphological variant 高不高兴 of 高兴不高兴 can be explained as phonological reduction, thereby fitting as a better morpho-syntactic structure for colloquial Chinese.
The above sounds right! yet the last part is not so scientifical as it sounds. Why do you use the word " must " ? do you mean that YOU ARE SURE that there is 100% likely hood that the hits of 高不高兴 are singnificantly higher in the written Chinese corpua than those in spoken ones. dr.xujiajin and cncorpus, why don't you both introduce more about how and from which corpus you both get the results? probably that will be more convincing.
secondly, expalantion of the statistical results will demand more than one or two linguistci theories before any meaningful conclusion to be drawn, in my eyes . more importantly, any theories generated from the research will be effective and living only when it can be used to explain the other similiar patterns in chinese. a little humble ideas about the thread!