Why has the USA suddenly sanctioned seven Pakistani companies?
Reading the news article that you linked:
The US government has announced sanctions against seven Pakistani entities allegedly associated with the country’s missile programme, says an official notification.
So it seems that these companies have something to do with missile technology.
Pakistan has always denied any wrongdoing in connection with its nuclear or missile programme, and Dawn was unable to verify if the names and addresses given were correct or in any way linked to the country’s missile programme.
Pakistan denies doing anything funky with missiles, there is no mention of sanctions against Pakistan, and we don't know if the companies have anything to do with Pakistani missiles. Pakistan could be lying, but then why isn't Pakistan being sanctioned? The seven companies could be doing business with another country (possibly through a proxy such that they didn't know, at least not for sure). For example, North Korea is currently working on missile technology.
Iran is another country that might be interested in missile technology. Both Iran and Pakistan are Muslim countries, so it would be natural for companies in one to be doing business with the other. Iran is not supposed to do this kind of thing under the nuclear deal though. Not sure if it would be a technical violation of the letter of the agreement, but it would be a violation of the spirit of the agreement.
It's speculation, but I would guess that these companies are doing business with North Korea or Iran. This is the United States calling foul.
Again, there are other options. Pakistan could be lying and we may see sanctions against them in the coming days. Or another country might be involved. Or we could be talking about small missiles, like Daesh might want to buy to use tactically against vehicles in Syria or Iraq. We won't know unless someone publishes more information.