Why did Estonia require some Soviet-era immigrants to apply for citizenship after independence?

Following the restoration of independence in 1991, Estonia faced the challenge of addressing the status of residents who had arrived during the period of Soviet occupation, which began in 1940.12

Massive immigration from Russia and other parts of the Soviet Union during the Soviet era had significantly altered the country's demographics, reducing the proportion of ethnic Estonians from nearly 90 percent before the war to about 60 percent by 1990. Because these nonethnic residents had settled in the country during the annexation, they were not automatically granted citizenship upon the restoration of the republic.3412

Consequently, these individuals were required to apply for citizenship through a naturalization process. A key requirement for this process was demonstrating proficiency in the Estonian language. This policy became a point of significant tension in relations between Estonia and Russia during the early 1990s.12