How can I understand them？
Please read the sentence firstly: A British man has been arrested by Massachusetts State Police after he allegedly started assaulting his wife on board a flight from London to Texas - forcing the flight crew to divert the plane to Boston.
In the sentense, the words ' a flight from London to Texas ', I think, is to explicate word ' board '. If it is so, why isn't there a word, ' that', between ' board ' and ' a flight from London to Texas '?
In addition, the words, ' forcing the flight crew to divert the plane to Boston ', means the behavior happened when the man was assaulting his wife. So why dose the sentence use it ' - ' ? Why isn't it written like this: ...after he allegedly started assaulting his wife on board a flight from London to Texas forcing the flight crew to divert the plane to Boston.?

Question

How can I understand them？
Please read the sentence firstly: A British man has been arrested by Massachusetts State Police after he allegedly started assaulting his wife on board a flight from London to Texas - forcing the flight crew to divert the plane to Boston.
       In the sentense, the words ' a flight from London to Texas ', I think, is to explicate word ' board '. If it is so, why isn't there a word, ' that', between ' board ' and ' a flight from London to Texas '?
       In addition, the words, ' forcing the flight crew to divert the plane to Boston ', means the behavior happened when the man was assaulting his wife. So why dose the sentence use it ' - ' ? Why isn't it written like this: ...after he allegedly started assaulting his wife on board a flight from London to Texas forcing the flight crew to divert the plane to Boston.?

Answer

Flight from London to Texas is in reference to the word flight not board.  So if you were going to use a comma it would go at:  ...a flight, from London to Texas, ...  I don't know if the dash could still be properly used after the comma.

Dash
A dash is an informal device telling the reader the that sentence is beginning a new but related thought.   When the new idea is done it should be closed with another dash or a period.

Answer

They used it in replacement of a comma. Some news articles due to and it's usually used to add emphasis to the situation.
I personally find it one of those tricky grammar things that are a pain to learn.

Answer

Sean, did you read the sentence firstly? There is not a comma between between ' board ' and ' a flight from London to Texas '!!!

Answer

Please read the sentence firstly: A British man has been arrested by Massachusetts State Police after he allegedly started assaulting his wife on board, a flight from London to Texas - forcing the flight crew to divert the plane to Boston.