Probably based on a perception that the IPO of the GP will wind up siphoning off profits which otherwise would have gone to holders of PAA. And it is clear that there will be money coming out of PAA which will get passed through to the new LP.
The GP already existed as a private entity. Taking it public doesn't change the cash flow pattern. However, I can believe that some PAA holders are moving to the GP for the higher dividend growth rate. However, as some have pointed out on this message board, the IPO was at a rich level as compared to other public GPs such as TRGP, KMI and ETE.