Here is enough background information [0] [1] [2] [3] quoted directly from people involved that should get you started. Unless Warren, Brazile, DWS and others are all wrong with their statements about it.
Neither Brazile, Warren, nor any of the leaked DNC emails provide even the slightest bit of evidence that any voting process was manipulated, any vote totals falsified, or anything else of that nature. They also do not show any evidence of any kind of adverse action against Clinton's opponents. There is a compelling argument that DNC staff were too partial to Clinton, who they viewed as the presumptive nominee. There is also evidence of animus against the Sanders campaign, although that animus must be viewed in the context that the Sanders campaign had previously _sued_ the DNC.
Party organizations involved in the primary process need to avoid even the appearance of favoritism. The DNC under DWS failed in this, and it also demonstrated other forms of unprofessionalism (such as passing around credit card information in unencrypted email!). Cleaning house was necessary. Furthermore, superdelegates had been a controversial issue in the last two contested nomination contest, and were clearly perceived by a substantial number of party supporters as undemocratic. Reforming them was also necessary. But none of that changes the fact that there is no evidence of any form of rigging of the nomination process.
If it's obvious there was some sort of corrupt manipulation of the process, it should be possible to point to at least one single state where there is evidence of electoral manipulation.
Insofar as those things provide any description of anything that might even loosely be described as a “system to allow rigging” it's the joint fundraising agreement between the Clinton campaign and the DNC.
Which hardly, even if viewed as such a system, can support your suggestion that such a system not only existed but should be suspected to still be in place.
Party organizations involved in the primary process need to avoid even the appearance of favoritism. The DNC under DWS failed in this, and it also demonstrated other forms of unprofessionalism (such as passing around credit card information in unencrypted email!). Cleaning house was necessary. Furthermore, superdelegates had been a controversial issue in the last two contested nomination contest, and were clearly perceived by a substantial number of party supporters as undemocratic. Reforming them was also necessary. But none of that changes the fact that there is no evidence of any form of rigging of the nomination process.
If it's obvious there was some sort of corrupt manipulation of the process, it should be possible to point to at least one single state where there is evidence of electoral manipulation.