Basically it goes like this;
Someone launches a new coin, and keeps a percentage of coins for themselves by writing a large sum of coins into the first few blocks, and mines those blocks themselves before launching the coin. They call it a premine. Sometimes a small premine is done as a way of paying devs, for giveaways, etc, but a lot of the time they're kept to do what is known as a "pump and dump".
A pump and dump means that they first build up hype for the coin so eventually it ends up on an exchange, then dump all of their coins for a big fat profit. Leaving the people who mined the coin with nothing, as after a large dump the coin is extremely devalued to the point of being worthless or even taken off an exchange.
If you look hard enough, you'll see which coins these are.