Basically, we know one of two possibly things:
1) Any alien can morph into a Queen
2) (AvP) A praetorian is required for a Queen to be born
But after watching Alien once more and due to the Alien's differentiation from Cameron's design, however design it might be, I've come up with an explanation. It's possible the egg was either 1) enhanced by the Pilots or 2) Quite possibly that alien could be a young Queen based on the deleted egg morphing scenes. Perhaps Young Queens, unfertile to produce eggs yet, can use egg morphing to protect herself.