Is it because of the large number of altcoins?
It won't be affected by them. The common thing that really have a huge contribution for this dip is the issues of regulating all exchanger and Mt .Gox members that are selling their btc in cheap price. If all of thesE will be done, then we see a bright side again.
It actually depends on the exchanger if they are following the official price or not, but since most of the exchanger are trying to synchronize their prices in the market then supply and demand is always the cause of it and we should not worry about the decrease in price since we can invest even more to earn profit.