You ancestors should know the situation better.

There was little knowledge about eastern part of Belarusian republic in the western part that was in Poland. Reading commentaries of people and articles of old newspapers in western Belarus people wanted to re-unite with eastern Belarusian Republic believing they’d have their own country where they will have Belarusian schools and newspapers. In Soviet Union, Belarus was a separate republic. In Poland western Belarus, western Ukraine, south-eastern Lithuania were Poland where many Belarusian, Ukrainian, Lithuanian schools , cultural centres, newspapers were closed. In 1930 and 1938  Hitler or Stalin were not threating Poland. Although, Hitler was preparing for war during 30s.  Poland was divided between USSR and Germany in 1939 with territories of eastern Poland were returned to Ukrainian, Belarusian and Lithuania republics. The line across which Poland was divided approximately the same where are the borders between Poland, Belarus and Ukraine today.  A good part of Bialystok voivodship including Bialystok city was part of Belarusian Republic till 1944 until Stalin gave it away to Poland to get some political favours from Polish government. Things were not that west Belarusians expected after they joined eastern Belarusian republic with Soviet authorities sending NKVD and trains to deport people in Siberia because they were deemed to be not loyal to the government.

Maybe your relatives left for economic reasons seeing many Jews leave the country?