The program that is used to build the universe has been changed to allow independent planets to have their own sector defenses.

When the universe is created and independant planets are placed in a sector they may also have their own sector defense in the form of fighters and/or mines.

Some sectors may have just mines, some may have just fighters, some may have both and some may not have any fighters or mines defending them. Some of the sector fighters may be toll and some may be set to attack. The more independent planets in a sector the more likely there will be defenses and a greater number of them.

So when you are just starting in a newly created game you had better be careful and scan before you move because that sector you are moving to might just have independent sector defenses.