How is it so easy for enemies to transport aboard a starship; for example when the borg transported abboard the Enterprise in First Contact?

In Star Trek First Contact the borg "somehow transported themselves aboard" the Enterprise when they knew there ship was doomed. Was it ever explained how this could have happened?

1. Why would the Enterprise have its shields down when it was just engaged in a huge battle with the borg and still was pursuing a borg vessel?


2. Even if the main shields were down doesn't a ship normally have its deflector shields up? Wouldn't this be enough to prevent the borg from transporting?


3. Is there really no security system in place to prevent intruders from transporting aboard (especially when all borg are bad)?