Final answer:
The cause of the plane crashing into the TV screen was the deliberate act of terrorism in the events of September 11, 2001.
Step-by-step explanation:
The question seems to be referring to the events of September 11, 2001, which involved the terrorist attacks on the World Trade Center buildings in New York City. The statement mentions seeing a plane fly into the first building and a TV set being on. The cause of the plane crashing into the TV screen, in this context, was the deliberate act of terrorism carried out by hijackers who took control of the planes and crashed them into the buildings.