As you can see in the chart, there was clearly a big dump of price in the highlighted area. These are generally caused by a big dump of coins or trading bots making poor trades. Big dumps come from whales and some bad whales often try to manipulate the market by a large dump or pump hoping to gain profit.

But that's a zero sum game.

You either win or lose. Often price manipulators lose money in this game. The other thing that comes to mind which could have caused this would be bots. Trading bots are programmed to sell or buy whenever x condition is met. For example, if price of x coin goes down 10% sell x number of coins. These bots are generally profitable but when a big dump or pump comes, they fail badly. They are generally made with the assumption that market will stay stable with somewhat volatility but unpredictable price movements can make the bot operators lose a lot of money.

There information are generally well known by crypto vets but I hope this will be beneficial for the newbies. Another good discussion about bots on r/cryptocurrency