The master private key is used to generate a master public key via elliptic curve multiplication.
I have read somewhere that knowing one or more private keys of the same seed may lead to the discovery of all the private keys.
I'm not sure exactly what this information is, but it's something close to that.
The case was related to the sale of one of your private keys (hardforks days) and keep using the wallet seed to generate other addresses/pvtkeys