The Soviets made an agreement that in the event of a war in Poland that there would be no aggression between Soviet and German military forces as long as they maintained spheres of influence. IE. The Soviets would not attack Germans as long as Germans did not cross into the region that was formerly owned by the Soviets.
Several weeks into the invasion of Poland the Polish administration FLED THE COUNTRY and left it completely undefended.
2 weeks after this occurred, the Soviets decided that moving into the region labelled as their sphere of influence was probably for the best as it would otherwise be occupied by the Germans as there was no central Polish defence force anymore.
When the Soviets moved in, the Polish Army maintained and official ceasefire with the Soviets, except for a few holdout fanatics who hadn't gotten over the Russo-Polish War.
Why is that? Because the Soviets weren't just taking the place over they were staying on friendly terms with the remaining Polish Army there which was leaderless and rudderless, moving up and defending the region from the Germans. This benefitted them of course by providing a buffer but it is absolutely not as simple as "haha they divided it up and planned it beforehand haha".
Only fucking liberals and fascists believe this shit. Don't be a fucking loser. Actually learn the Marxist perspective on the history.