For me, those brain worms are often bits of story, bits of poem that just won't grow. They're like malaria: infesting my mind but often hidden, rising every once in a while to be obsessed over in a fever of impotent creativity.
One such brain worm I am infected with (I am a veritable Typhoid Mary of literary parasites) is a fragment of poem that has been hounding me since the mid-90s:
Bright Orion striding 'cross the skyThis fragment seems to emerge nearly every year right around mid-winter, and I have at least a dozen discarded efforts at completing it to prove it. I love the image, and I like the cadence, but sadly it will not grow. And yet, it demands attention.
Looks down upon the fields all white with frost 
When I was in Canada, the persistence was perhaps understandable - winter is the best time to view Orion, and of course Manitoba winters are an excellent example of the land being frosted over with white - those crusty layers of wind-sculpted snow and ice...
But here in Japan? Particularly here in Tokyo, where a couple of centimeters of snow can bring things to a screeching halt, the horizon is obscured by a forest of sky-scrapers, and the sky is so hazy with pollution and heat that I can rarely see more than ten stars at once? (whoops - nine: that one was a plane)
Short of assuming that the very fact it's winter triggers the brain worm's emergence, I'm at a loss. There's no explicit connection to Christmas, for example, and Orion is not particularly easily observed in this part of Japan even in mid-Winter.
So what triggers the brain worm episode?
I have no idea. And this is pretty much the definition of brain worms for me: little fragments that rise up, obsess me, and then disappear seemingly for no particular reason at all. And the worst part?
When the worm is active, it becomes difficult to work on other creative projects - and yet the worm itself never seems to go anywhere.
It just gets in the way for a while, then hides until the next time.
1. I sometimes wonder if the seed (egg?) for this particular brain worm was planted by exposure to Virgil:
Then wakeful Palinurus rose, to spyThe face of Heav'n, and the nocturnal sky;And listen'd ev'ry breath of air to try;Observes the stars, and notes their sliding course,The Pleiads, Hyad, and their wat'ry force;And both the Bears is careful to behold,And bright Orion, arm'd with burnish'd gold. (Aeneid - Book III)But it's not as though I have been saturated with Latin classics - even if this is the seed, it's strange that it would have such a long-lasting (and unproductive...) influence.