I'm probably on a watch list
What do you mean when you say that you are probably on a watch list?
I was making threats about crashing the stock market.
That is a bad thing to do. It is not okay to be a threat to others. People will not be able to trust you in the future.
It's just a joke. I was joking around.
Well you are still threatening to mess with something very important. This could have some implications on you to be honest since the stock market is something that most people would prefer to be running smoothly.