Why is news said to be "breaking"?

The word breaking in this context means to

undergo a change or enter a new state, in particular ... of news or a scandal [NOAD]

Etymonline has this interesting tidbit to offer about break:

Meaning "to disclose" is from mid-13c.

That meaning is the flip side of the way you used it, and it is a transitive verb: "He broke the news to me that .." I believe that got modified to "breaking news" by extension.

Breaking news here means that it is new.

We can say 'the breaking of day' or 'morning broke' meaning that it is the start of this event.

'We broke the story' means we caused it to become a new event by being the first to spread it.

"Breaking" news, is "fresh" news that is happening AS WE SPEAK. We "broke" the story, means, we caught "it as it was happening" (Think of "breaking new ground.")

In English grammar, it is a reference to the "present progressive" tense.

To break the news to someone most probably is the image of the medieval messenger who bears his message as a roll of parchment, rolled together and sealed. When he comes to the king he breaks the seal and reads the message to the king. He breaks the news to him, actually he breaks the seal and reads the news to him. This two-part expression was shortened, with to break from the first part and news from the second part. Astonishing that etymonline does not mention to break the news to someone.