How did Pattern know about Szeth?

[Through The Living Grub He/Him]

Before Szeth appears to try to kill Dalinar, we have a scene with Shallan where Pattern is mumbling to himself that Szeth is coming, or something to that effect. How did he know this? Was it a writing decision that Sanderson made to increase the tension by making the spren act weird, or spiritual mumbo jumbo about Shallan's double bond giving Pattern knowledge of this event?

[alder24]

Neither Syl or Pattern reacted to Szeth during that night, they both sensed the presence of Odium nearby because at that very time Eshonai was acquiring the Stormform and Odium was watching her. That's what they both sensed. The chapter in which this happens is called "the one who hates," which is a reference to Odium and in a dream Kaladin had at the beginning of that chapter, he was talking to the Stormfather who has shown him the incoming Everstorm and talked about someone who's coming for Kaladin. We're left believing that the Stormfather was talking about Szeth, but what he spoke of and what Kaladin felt in the vision indicates that the Stormfather meant Odium. The very next interlude I-5 after those events is the one in which Eshonais transforms into Stormform and has a short talk with the Stormfather, so we know this happens at the same time and that's why the Stormfather suddenly warns Kaladin. It's nearly a coincidence, or a stroke of Fortune that Szeth tried to kill Dalinar at the very same moment as all of this was happening. 

WoR ch 32:

Quote

AH, the Stormfather said. SO IT WILL END.
“What?” Kaladin shouted into the winds. “What changed? I feel—”
HE COMES FOR YOU, LITTLE TRAITOR. I AM SORRY.
Something rose before Kaladin. A second storm, one of red lightning, so enormous as to make the continent—the world itself—into nothing by comparison. Everything fell into its shadow.
I AM SORRY, the Stormfather said. HE COMES.

[...]

Syl looked into the sky, then turned her head sharply over her shoulder. “Kaladin. Something is wrong.”
“I know.” Syl spun about, twisting this way, then that. Her small eyes opened wide. “He’s coming.”
“Who? The storm?”
“The one who hates,” she whispered. “The darkness inside. Kaladin, he’s watching. Something’s going to happen. Something bad.”

[Returned he/him]

My interpretation was always that Szeth's coming was a major move by someone relevant enough in the Cosmere (whether Odium, Taravangian, Szeth, or even possibly the Honorblade) around enough other relevant people (Dalinar, Kaladin, Syl) that it had spiritual reverberations. The Syl also is aware of Szeth's impending arrival, if imprecisely, so it's not just Pattern and therefore probably not related to the double bond nor Heraldic parentage. The Stormfather knows and specifically tells Kaladin about it, though we know he can perceive via the Highstorms and so he has a clearer explanation.

In my mind I connected it with foreboding sense that preceded the coming of the Everstorm, but in light of this question I'm not sure they're so directly comparable. I think we could probably rig together an explanation that is lore-friendly enough to be acceptable, but if I had to wager on it I would place my bet on it being a narrative decision to increase tension (as you suggest).