This is starting to remind me of those discussion with jorge, when he wants to find out the definitive "reason" for each rally or crash
Keep in mind, I believe that technicals rule the market, or maybe the slightly weaker claim: market sentiment (which doesn't require external reasons) heavily factors into the interpretation of the fundamentals, to the point where it makes sense to say "technicals rule the market".
So, the answer is probably somewhere in between cyclical market sentiment, professionalization of mining (with higher ratio of coins sold vs. coins held), professionalization of trading (with more emphasis on short term profits), lasting negative influence on user confidence from mtgox failure (something that this forum is very reluctant to accept, imo), and uncertainty if growth of user adoption is accurately captured by an exponential growth function (I know, hashrate very clearly is, but that's not the same as user adoption).
I also agree with you on the sentiment that you can't just reduce a complex situation to an almost binary statement such as "mining is the cause".
This is almost as bad as "it's the gox coins, it's exchanges manipulating, it's the government".
So the supply of bitcoin is increasing around 14% a year at the moment? If during that time bitcoin adoption grows by 14% the effect should cancel out.
Also why is it always assumed that the coins mined are immediately (well, after maturing of course) sold?
You'd think that a reasonable miner would try to get the most out of their coins if they wanted to sell them.
Dumping bitcoin to the ground is a totally counter-intuitive move (let's leave out the conspiracy theories of miners trying to force others out of the business).
I don't know a lot about trading but from what I gather shorts are up significantly at the moment.
It is the general market sentiment that things are bearish so people try and profit from that.
This sentiment can and is creating much more downward pressure than miners could ever make