You have to consider that miners may follow basic economic theory too (that people will hold on to their money if they expect it to appreciate)
Hoarding by miners, plus heavy buying by speculators will push the bit coin price further up, providing nothing catastrophic impacts upon the bit coin economy.
But miners have running costs and are spending their time on it. Unless they are hobbyists or have loads of savings to burn on this particular kind of BTC "buying" they would have to sell at some point.
Assuming that the big miners are semi-professionals and thus depend on BTC income is not that unreasonable.
The other major sellers would be BTC millionaires slowly living off their BTC treasure, I am guessing this flow is of minor volume.
Another part of my theory/prediction is the "least resistance"-idea: BTC would tend to flow to those who believe in the idea and are saving it.
Thus larger and larger amounts of BTC get "locked up" in savings accounts.
However new BTC, mined coins, statistically should be more likely to be in the hands of sell-minded people and will thus set the price.
Speculators may sell, but their money is likely still at mtgox and they will usually buy back in at some point so at the end of the day they are simply "buyers".
Only once normal BTC holders/believers also become spenders in a greater BTC economy will all these dynamics change.