Sources of upcoming market-moving events?

Sort of a broad noob question, but there's some flesh to it. Please bear with me.

Here's the DOW from yesterday – https://i.redd.it/8v2rhp55chpa1.png

There's a first movement related to Fed's rate announcement. This is obviously expected. Then there's a sell-off 1.5 hours later and the cause of that is – to me – unclear. Some say it's linked to Yellen statement on “no blanket bailouts”, some say it was due something else, etc.

I may be way off, but I'd imagine that larger investment firms would maintain a list of possible market-moving triggers, have trading decisions ready for possible outcomes, will keep a close track of these events and react as per their plan.

Also, based on the rate of yesterday's drop, I would guess that these “watchlists” and trading decisions would be fairly similar between the firms. That is, it'd be more of a common knowledge in professional circles.

Hence the question – how could someone not connected to the Wall Street tap into this? What would be good sources to follow to pick up on this sort of information?

TiA!