CREATION OF THE MID-SECTORS, OR THE SECOND EXPANSION
It is worth noting that in the history of colonisation, there was never a total end to colonisation between the First Expansion and the Second Expansion. As the First Expansion slowed to a trickle and officially ended, colonisation would still happen, in an often haphazard and more genteel way than seen previously.
Much as with the First Expansion, the Second Expansion was triggered by a great event. This event was the Droid Intelligentia, the Droid Wars, which led to a hardening of views against robotic slavery but also saw the emergence of the humanist and the borgite divide.
The distraught, disgusted and weary human race, once again seeking a new beginning, began to reach out again. It was fuelled by better technology, with the ships able to travel further, and the people able to multiply even quicker in the variety of ways they now had. Robotic droids were no longer needed; it was cheaper to grow a new human than create an artificial drone (that may turn on you and try to kill you if it became corrupted).
As the human race expanded rapidly, fresh from the Droid Wars, new territories were created at pace. As the expansion went in all directions, it was the richer north and the cultural south that saw the greatest, prolonged period of expansion. At the height of the expansion, the father of the man who would become the True Emperor came to rise in the galactic north, and tried to reunite the human race. As fighting broke out anew, the Second Expansion slowed and eventually came to a halt.
As before, even with the wars gathering apace and spreading from the north towards the core, the colonisation still continued, albeit much more slowly. The Mid-Sectors and the Far-Sectors were formed, the latter to be the foundation of what would become the greater region of space known as the Boundary. As the father of the True Emperor died, in the period shortly before the True Emperor began to conquer and unify the human race, the Second Expansion ended.