[davem]

Also reminds me of Halbarad's statement at the door of the Paths of the Dead:

'This is an evil door,' said Halbarad, 'and my death lies beyond it. I will dare to pass it nonetheless; but no horse will enter.'

. Why is this significant? Go back a bit to Halbarad's first appearance:

And Aragorn said to Halbarad: 'What is that that you bear, kinsman?' For he saw that instead of a spear he bore a tall staff, as it were a standard, but it was
close-furled in a black cloth bound 50 about with many thongs.
'It is a gift that I bring you from the Lady of Rivendell,' answered Halbarad. 'She wrought it in secret, and long was the making. But she also sends word to you: The days now are short. Either our hope cometh, or all hopes end. Therefore I send thee what I have made for thee. Fare well, Elfstone!'
And Aragorn said: 'Now I know what you bear. Bear it still for me a while!' And he turned and looked away to the North under the great stars, and then he fell silent and spoke no more whilethe night's journey lasted.

. Halbarad bears the banner to Aragorn, & foresees his own death (he does fall on the Pelennor).

Look at this:

A triangular banner appearing to depict a bird (possibly a raven) appears on coins minted by Olaf Cuaran around 924. The coins feature a roughly right isosceles triangular standard, with the two equilateral sides situated at the top and staff, respectively. Along the hypotenuse are a series of five tabs or tassels. The staff is topped by what appears to be a cross; this may indicate a fusion of Norse pagan and Christian symbolism. The raven banner was also a standard used by the Norse Jarls of Orkney. According to the Orkneyinga Saga, it was made for Sigurd the Stout by his mother, a völva or sorceress. She told him that the banner would "bring victory to the man it's carried before, but death to the one who carries it." The saga describes the flag as "a finely made banner, very cleverly embroidered with the figure of a raven, and when the banner fluttered in the breeze, the raven seemed to be flying ahead." Sigurd's mother's prediction came true when, according to the sagas, all of the bearers of the standard met untimely ends. The "curse" of the banner ultimately fell on Jarl Sigurd himself at the Battle of Clontarf:

Earl Sigurd had a hard battle against Kerthialfad, and Kerthialfad came on so fast that he laid low all who were in the front rank, and he broke the array of Earl Sigurd right up to his banner, and slew the banner-bearer. Then he got another man to bear the banner, and there was again a hard fight. Kerthialfad smote this man too his death blow at once, and so on one after the other all who stood near him. Then Earl Sigurd called on Thorstein the son of Hall of Sida, to bear the banner, and Thorstein was just about to lift the banner, but then Asmund the White said, "Don't bear the banner! For all they who bear it get their death." "Hrafn the Red!" called out Earl Sigurd, "bear thou the banner." "Bear thine own devil thyself," answered Hrafn. Then the earl said, "`Tis fittest that the beggar should bear the bag;'" and with that he took the banner from the staff and put it under his cloak. A little after Asmund the White was slain, and then the earl was pierced through with a spear.http://en.wikipedia.org/wiki/Raven_banner

Honestly, my feeling is that Tolkien is drawing on incidents like these, & they remain ultimately mysterious & unexplained because the intent is to impart a flavour of 'Northerness' to the story.