User blog comment:Xilinoc/Ch. 668 - BIGGER,FASTER,STRONGER/@comment-1970092-20160331203556/@comment-3423279-20160404060413

Or, a Shinigami has to be "worthy" in some manner to hold a Bankai.

I mean look at Renji, even though he attained Bankai he still wasn't worthy of it in his Zanpakutou's eyes, so the spirit itself seems to have more control over the release more than one would think.

If Yachiru is indeed Kenpachi's Zanpakutou spirit then she might view him as worthy of Bankai at this point.