1. They're already wealthy people.
2. They have loads of VC money
3. They used to make what is likely to be ridiculous amounts of profits selling mining gear.
I have heard the argument "he is rich therefore he doesn't need more money" used to introduce a couple of egregious scammers and thieves. Even in this context, it is a bogus argument: usually, rich people are rich because they always want to make more money that they already have.
KnC seems to be "selling" some of their mined coins in the form of the XBT Tracker One Electronically Traded Notes. Those are issued through NASDAQ Sweden by a company somehow connected to them. As I understand it, KnC keeps the bitcoins and sell IOUs that promise to pay the holder whatever the current BTC price will be at the time.
The buyers of those notes may be a different population than the people who buy at the exchanges, so those "sales" may not impact the price immediately (as they would if KnC were to sell the BTC on the exchanges). However, if many holders decide to redeem the notes, KnC may be forced to sell the BTC to raise the money for that.
The notes that they issued and sold so far correspond to ~16'000 BTC. I am too lazy to compute how much that represents compared to their mining (~8% of the hashrate presently, which means ~12 × 25 BTC every day).
It would be perfectly logical to consider $400 a low price a year ago, and $230 a high price today...
Anyway, miners have an obvious incentive to lie and hide their sales. Whether they hold or sell immediately, they very much want everybody else to buy and hold.