Why is your army in Sector War 144

The veil of warp storms has been lifted and the unknown awaits in sector 144.  The question we all have to answer is what brings our armies to the unclaimed wilderness of 144.  Below I'll discuss some options folks could select as their army's reason for being here.

1.  Imperial Crusade Fleet
With a possible hundreds of worlds within the sector, many habitable, the Imperium of Man has dispatched a modest crusade fleet to claim the sector for itself.  While the fleet will include many explorers and catalogers it will also include war formations intend to destroy any opposition to the crusade fleet and retake worlds previously owing imperial allegiance.  Other wrinkles might be your imperial force might have joined the crusade fleet to complete some objective for their own.  Perhaps a space marine chapter used to have an outpost on one of the multitude of worlds lost in the storms six millennium ago and are here to see what has happened to the post.   Perhaps an imperial force has been co opted by the inquisition to recover some relic or some such.  Perhaps their mission is secret and known only to themselves.

For the non imperials the crusade fleet might be your army's reason for being there as well.  Perhaps Dark Eldar Corsairs seek to destroy the crusade fleet while it is isolated from outside aid.  Perhaps a Hive fleet Tendril has been stalking it since it was detected.  Perhaps a mass translation of ships out of warp has worn thin the fabric of reality in an already destabilized zone and a massive daemon incursion is about to take place.

2.  Looters
Xenos, Chaos, and Imperial alike by be in sector 144 for this purpose.  Artefacts taken from worlds once embroiled in the warp can be valuable, let alone the possibility of riches to be taken from the Imperial worlds cut off and unsupported by outside help.

3.  Locals
The planets of 144 weren't all uninhabited when the warp storms commenced.  Imperial worlds, Necron Tomb Worlds, Tyranid Hive Fleets, Ork worlds,  entire Space Marine chapters, and more could have been caught up in the storms.  Are these forces still loyal to their home factions, or have the last six thousand years of forced separation made them value their autonomy too much?

4.  Lost
Funny thing about warp storms, they don't follow any of the rules of physics.  A warp disturbance in sector 144 might have been in two places at once, and the unwary, unlucky, or the uninitiated might have found themselves flung thousands if not millions of light years off course into an unknown sector filled with stranger stars when the storm dissipated.  This lost mechanic is mostly intended to give the Tau a way to be in sector, but is perfectly suitable to other Xenos, Imperial, or Chaos armies as well.

So those are the basic story hooks to get your army into the war.  If you have your own idea, GREAT, you certainly don't have to use any of these four to get your army involved.  The storyline for Sector War 144 is very much a sandbox to be created by everyone involved.