At Mr. Wayne's suggestion, I'm pasting this here from another thread, it's definitely a theory.
We were talking about Brandon Stark/ Rickard Stark's executions...
(Brandon was an)
According to Lady Dustin, this is true.
And hot-blooded, for sure.
He went flying off to King's Landing all half- cocked and brought much more grief to the Starks. In fact, I think Lyanna sent a raven explaining what was the real deal with Rhaegar, and Brandon either read it as Rhaegar's words in her writing (ala Cersei/Sansa's letter to Rob much later) OR
he was miffed at her rejection of the Lord of Storm's End/ ramifications for soiling the Stark name.
I think Brandon was the only one who saw Lyanna's letter, and made up the kidnapping himself to protect the family name.
This post was edited on 9/21 at 12:41 pm
Since posting this, I checked and found that Brandon was already en-route to Riverrun for his wedding to Cat when he got wind of Lyanna's..absence. Don't know if he intercepted a messenger on horse headed North to Winterfell or just how he heard about it, but it sorta bolsters the suggestion that he heard it first and could have been the only one to read the actual (sealed) letter. I think he destroyed it, sending his own version of events North as he rode further South to King's Landing.
IMO, Lyanna was too smart to trust such news (breaking betrothal to Robert) to a raven, if intercepted or killed coincidentally, the news in wrong hands could be big trouble for Winterfell.
This post was edited on 9/28 at 11:01 am