How the egg got on the Sulaco ........

Simplez ,
While the queen was stomping around the cargo bay looking for newt and ripping up floor tiles , the exhertion made her fart one out and it got hidden in the floor duct newt was hiding in.

I assumed the queen alien was in an area where the landing gear retracted into during transport to the Sulaco and laid an egg in there. It got interrupted doing that exact thing and might have been holding an egg inside ready to pop out when it had a moment. I've had chickens and one of them always held it in until I let her out to go to her favourite spot in my yard to lay literally a moment later. They would have had a few minutes of time while they landed, the airlock closed, they shut down the engines, they might have a moment of relief and calm as it all goes quiet and they are exhausted, then open the ship's door and walk out to have that brief discussion before all hell breaks loose. If the egg has all that slimy sticky stuff holding it down at the base then it may not have been sucked out into space when Ripley opened the airlock. Why the egg opened by itself (when proximity or movement is meant to trigger them) I can't explain. Could be that sometimes they just randomly open.

Ummmm the Queen just laid a few eggs while she was on the Sulaco. How is that a plot hole?

I would have prefered if Bishop brought them. People will dislike Bishop turning evil, but his actions in Aliens could be interpreted as rising the probabilities to bring a specimen to the company. But it is easier to imagine that this movie never happened. Much relaxing thought.

Its not a plothole. The ending of Aliens has a SFX of an alien egg opening. Its already established that there was an egg on the Sulaco.

Here are 2 possible reasons why:
1) Bishop brought one or more on.
2) The alien queen brought one or more on

That could mean that the WY robot Bishop wasnt really trustworthy after all.

Probably the Queen laid some eggs when she boarded Sulaco and before attaching the crews. She was a smart bitch. Later when everyone went to sleep, the eggs may have sensed slow hearbeats/breathing and then released the facehugger.

Or we have to put Bishop in a grey area. Even though he was a good robot, he still had protocols and carried some egg samples for submitting to the company.

This is the best one can think of!