>Who decides when a stock get's halted to stop it from tanking?

It's partially automated and partially up to the primary market.

Each exchange has circuit breakers for detecting large drops in both the stock's price in a short time period and a drop in liquidity. These are also known as non regulatory halts.

Additionally for material news leaks like this there is a mechanism to manually halt trading. These are known as regulatory halts.

while there are any number of reasons a trading halt can occur, few of them are particularly beneficial to zynga. In this case the most likely candidate is public disclosure requirements by the SEC. It is tricky to announce things uniformly across a multitide of outlets, sometimes a press release will go out early and the company will ask for a halt until they can get the rest out. In this case it looks like marketwatch had the news at 2:48 but zynga's official press release didn't hit until 2:52.

Stocks get halted all the time.

Sometimes for news, sometimes due to a fat finger, sometimes due to market volatility.

What normally happens in the case of news is that the stock will be halted before the news comes out and then the stock will reopen sometime after the news so that everyone has a chance to digest the news.

In the case of a fat finger or market crash things are a little different. In that case trads can be undone, meaning that if you bought as the stock crashed you could get your trade busted. After the trades have been undone, the exchanges make sure there is adequate liquidity on either side of the book and trading resumes.

This second case is a bitch for automated trading systems as we(the designers) don't often take this into account and even less frequently get a chance to test this case in the wild.

It often happens after huge news, and it's meant to give investors time to digest it.

I believe the last notable instance was after Jobs died, AAPL was halted.

I think it can be triggered automatically when a major shift happens in a stock. I remember a few of these back in the 2000s when things were crashing left and right.

Stock trading can be halted after big news is released to prevent a panic. It gives the traders enough time to cool down after hearing bad news so they don't all sell at once.

If there's any chance to prevent a crash, a stock halt will help. The crash can still happen, of course.

If a stock falls by x% in time t, it's automagic.

It is typical to briefly halt trading in a stock, if a company makes an announcement (or undergoes an unexpected external event) likely to have a non-trivial effect on its stock price. This is to allow time for significant news to disseminate into the marketplace. For this reason, you will frequently see companies make major announcements outside of trading hours.