Lot's of reasons that cause the bitcoin to go down at the moment:
1.) In april there was money moved out of cyprus with the help of bitcoins. There was a high demand that caused the price to go up. But this has ended. So less demand and value 2.) Until the end of may the Winklevoss Twins bought lots of Bitcoins for their Bitcoin fund (they claim that they have bought bitcoins for 11 million USD. In april they owned 1% of all existing bitcoins). They probably stopped that which means there is now less demand and lower value. 3.) ASIC Miner is now doing 20% of the hashrate, plus the other ASIC mining people driving the difficulty up a lot. This is the cause that lots of GPU Miners stopped mining bitcoins. But when they don't mine bitcoins anymore, they don't spend them in shops like bitmit or bitcoinpride, they don't place bets or play poker with bitcoins anymore... Which means less demand and lower value too. The ASIC miners seem just to dump their bitcoins to get their investment back or to buy even more ASIC miners.