That's because in some peoples' minds the BTC/USD is inversely tied to difficulty.
Also, the BTC/USD spike is biggest on mtgox and should thus be disregarded / discounted.
Difficulty has generally followed price whenever price has increased (though not necessarily when price has decreased). This makes sense intuitively because an increase in price increases the viability of mining up and until the subsequent increasing difficulty then reduces the viability of mining.
However, the current exponentially growing rate of difficulty is driven both by the order of magnitude increase in BTC / fiat and by the (multiple) order of magnitude increases in hashing rate from the use of ASICS. This may prove to be the fastest increase in difficulty that Bitcoin ever goes through.
Whats never been clear however is that the increases in difficulty then eventually result in further increases in the BTC / fiat rate. In fact, the opposite has been true with Litecoin, where difficulty increased until it wasn't sufficiently viable and then decreased slightly, while the price of LTC itself has been steadily decreasing since April. This is actually very similar to what happened to BTC in fall 2011.
Regarding the price spike, BTC is up across the board by at least 20% in the last 5 weeks (discounting gox). It seems there is growing consensus that the BTC price bubble has finally ended, and given its relative magnitude to the 2011 summer bubble, its very possible that it has. More interesting (to me) is that the exponential growth in difficulty has nearly doubled the temporary BTC inflation rate for a period of now nearly 3 months. That will (obviously) not continue over any substantial period of time for reasons that should be self evident.
Think about what that means for what is to come. I know what I'm doing.
