It's because everyone saw what happened last week, with the price skyrocketing - and consequently they're expecting the same thing to happen again now that the rewards for mining have reduced....... no whale has stepped in to bring the price up, so consequently the weaker traders are starting to panic..... "ooh, it's not going up like I thought - bail now" kinda thing.
It's a great ploy by the big money men - make people believe the price will go up when the mining reward drops, and when THEY don't inject a load of BTC into it, they watch the weak bail out.... once it goes low enough they'll buy everything up once more, and the sellers will be kicking themselves - again!
Same story, diffrent coin:D