So there's this psyker lady who has precognition while fabricating the primarchs while they're still in their stasis tubes as proto-infants. She realizes what the Emperor is going to do with her children, using them to wage war across the galaxy for his own desires. Using them as tools, she also sees the death and destruction of many of her sons. To save them, she
SENDS THEM INTO THE WARP SO THAT WAY THE EMPEROR WILL NEVER FIND THEM AND CAUSES THEM TO BE LOST TO HUMANITY FOR DECADES, HER OWN PRECOGNITION NOT GIVING HER THE FORESIGHT TO SEE THAT SHE WAS THE ONE DAMNING THEM TO A FATE SHE WANTED TO PREVENT BECAUSE SHE'S A FUCKING RETARD
LIKE INSTEAD OF LETTING THE EMPEROR RAISE THEM AND LEARN TO ACCEPT THEM AS HIS CHILDREN SHE INSTEAD ENABLES HIM TO REDUCE THEM TO JUST LIVING WEAPONRY THAT ROWBOAT GORILLAMAN COMPLAINS ABOUT WHEN HE VISITS THE CORPSE-EMPEROR AFTER HIS OWN RESURRECTION
Fortunately after this lore-breaking bullshit another writer quickly tacked on that the Emperor has precognition and knew the Horus Heresy was going to happen anyways, so he allowed her to abduct his children since it was the least-damaging result of creating the primarchs.