Here's two examples:

You have inside information that evan will no longer be developing darkcoin, you know that there's a good chance the market will react to this,
so you dump all your darkcoins. If you have a lot of darkcoins, dumping that many darkcoins could trigger a downwards trend ( although unlikely ).

You have inside information evan has just discovered something revolutionary, or that Darkcoin will be used instead of Bitcoin on SilkRoad, you know
that this could have an effect on the value and could potentially trigger a bubble, you could then buy a shit load of coins ( which could also have an effect
on the value ).