They generally don't.
To make that type of statement accurately you would have to go through a very rigorous scientific anylasis. I imagine news pundits don't have the background or resources to do that.
This is especially true if you want to differentiate between random events that may not be correlated at all. For all intents in purpose you can't prove that one event caused a move in the market, or that move in the market was going to be there anyway and that "trigger" event just happened to be broadcasted at the same time. It could be entirely possible that no one with a certain position was even aware that event happened in the first place when they made their actions.
For example consider someone who has a position in stock A. Lets say they are up 500% on their position to date. Also assume they have a large position (Something like a million shares). This person needs to sell quickly, they need the cash for a better opportunity or something. How do you get out of a position quickly? Give buyers a reason to buy, sell the stock for cheaper then the current market price. Say they decide to sell at a 10% discount to get the money cause they don't feel like waiting a week to get out of the position while they wait for enough buyers to sell out their position. At the exact same time that this person puts through the trade, news comes out about president Trumps tariffs, and how they may effect the general sector that stock A is a part of. Because you are selling a large position you are moving the market price, and some retail investors see that stock A is going down in price. The retail investors get scared and sell off all related stocks. If you have enough retail investors getting scared and selling they can move the rest of the prices. For all you know 2 people watched the news break, and those retail investors wern't even aware that Trump made the comment.
As an outside observer can you realistically prove that the Trump announcement caused the market to move down, or can you disprove that someone is just in a hurry, and a bunch of retail investors got scared? If you went through enough trouble sure you probably could. But do these pundits, or their team go to that trouble? No. They just make a story of what is obvious to them, they could be right, they could be completely wrong.
They can usually get away with making the comparison because "logically" it makes sense. If Trump announces tariffs on a country, and that company with stock A does a lot of bushiness in that country, saying that Trumps announcement caused a run in the sector probably isn't a bad explanation. But they could also be completely wrong, and no one would ever know that. Logically it seems like a good enough answer so people usually don't care enough to question it.