It is rude to take someone else's place without permission.
Tom left his seat at the game a while ago and hasn't returned, Lucas looks at the seat.
Lucas wants to sit in the seat.
Lucas asks the other people if the seat is vacant, they reply that thier friend Tom left.
Lucas just walks up to the seat and sits down in it.
Lucas takes the seat and enjoys the rest of the game.
Tom's friends all start shouting at Lucas for his rudeness at simply grabbing a seat.