The Quori that became the Kalashtar were good-aligned refugees that fled from the plane of Dal Quor, to escape the evil oppression of the ruling faction of their plane.
They shouldn't have been able to cross over though, because Dal Quor's influence upon Eberron was severed entirely by the giants of Xen'Drik forty thousand years ago, after the evil quori invaded eberron in a war to dominate another plane.

But about 1,800 years ago, the first refugee group of 67 Quori came to Eberron by shedding their bodies in Dal Quor to become living beings of psychic energy. They crossed over and inhabited the bodies of willing vessels, creating the very first Kalashtar.
Over the centuries, the Kalashtar became their own distinct race separate from the humans they first inhabited, and all kalashtar are connected to their ancestors. When they sleep, they do not dream, but experience the memories of their quori ancestors.

The evil aligned Quori also figured out a way to get back into Eberron, taking a cue from the Kalashtar.
They did not inhabit willing vessels, but influenced the dreams of eberron mortals to do their bidding, and over 300 years, influenced them into breeding descendants that made ideal mortal beings for these evil quori to dominate and psychically control from their home plane of Dal Quor.