It's not even really a "new ability," just an extension of what we've already seen. The eggs obviously start out small and develop over time within the queen's ovipositor, right?
My theory just speculates that tearing herself free of her egg sac left the queen with a gaping hole in her abdomen and her "ovaries" exposed. As she fought Ripley in the Sulaco's hangar bay, at least one of her premature eggs fell out of this abdominal cavity and oozed through the floor-grates where it congealed into place and took root.
In keeping with the aliens' physical hardiness, this proto-egg was able to survive outside of the queen's "womb" and even grow on its own to full-size during the Sulaco's three-week journey back to Earth. Mind you, this process would be relatively slow compared to the aliens' usual accelerated growth rate.