The board was previously convened and dismissed due to misconduct by some of the board members. It turned out that some of the MCPOs were communicating with people being considered for selection.
The board is reconvening with the addition of post-command CDRs and CAPTs (O5s &O6s) and limiting the E9s to those in major billets is to provide oversight by individuals with authority to issue legal orders to those in the deliberation tanks.
This isn't the first, second, third or even fourth or fifth time in the last 10 years that a senior enlisted board comprised mostly of senior enlisted had a problem like this; like it or not ... more officers being brought into the process was brought on by the missteps of the senior enlisted participants on these boards.