It's not so much manipulators, it's how market makers operate in today's digital world. 50 years ago, you had people physically buying and selling on the market floor. Say I have a share of ABC Corp. that is currently worth $100. News comes out that ABC's profits unexpectedly fell last quarter. Now, I try to sell my 1 share for $100.
If I manage to sell it fast enough (before other people have the news), I will get nearly $100 for it - but after news spreads, people will offer less and less until the stock reaches some equilibrium point. This move will happen quickly - a matter of hours or even minutes. During that time, there will be a constant flow of orders happening and high trading volume.
In the last 15 years, those "market makers" have been replaced by automated systems, and the internet has transformed transactions that used to take minutes into ones that take microseconds (not even 1 millisecond!). How important is speed? Trading firms pay millions of dollars to be physically located nearest the exchanges.
So let's say ABC puts out some bad news today. All of these automated systems see the news immediately, and they place their orders, getting in and out of the market before you and I have even seen the news. The downward move that used to take minutes now takes a fraction of a fraction of a second. This happens so fast that the sell offers rapidly exhaust the buy bids and the stock price crashes to whatever the lowest bid happened to be. Even if only 1 share changes hands at that price, it'll show up as the current stock price if only for a short time.
That's what happened in the Flash Crash. Prices fell so quickly that most trading firms detected a problem and shut off their computers - the market literally ceased to exist for a short time. Prices lurched back and forth several points at a time, then rapidly recovered as the traders re-entered the market. Yes, some stocks did plummet 50%+, but only in very small amounts at the bottom of the bid/ask chain.
Sorry for the long reply, but I find market moves like this fascinating. It's a fundamental issue that has yet to be dealt with, and one that will continue to happen as we move forward technologically.