This is to be expected.
With the help of sites like dustcoin or coinchoose, it's very easy to calculate which coin that gives the most profitability.

People mine that coin and then instadump it on the market to get btc.

And there's that Vircurex hacking incident.
The attacker gains root access, and gets vircurex's hot wallet.
I think 30-60% of that stolen alt coin is dumped right now at BTC-E.