I think there are a number of reasons:
1. Fear----they want to create fear in the Indian community
2. Irrelevance----APNU/AFC represents a break from traditional politics in Guyana (well, at least a fair attempt at a break). If this occurs, PPP is dead as it is presently constituted. They will become irrelevant because they will be seen as an Indian party. They will be forced to change, most likely some of them will emigrate.
3. Race----PNC v PPP has always been Coolie vs Black. Indians are more than Afros, thus PPP don't have to come up with new ideas, just use the age old system of us vs them.