Assertion: Before its disintegration in 1991, the USSR consisted of 15 Soviet republics.
Reason: The independence movement began in Lithuania in 1988, and later spread to Estonia and Latvia.

Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Before its disintegration in 1991, the USSR consisted of 15 Soviet republics.
Lithuania was the first republic to declare independence from the Soviet Union by the Act of March 11, 1990.
The independence movement began in Lithuania in 1988, and later spread to Estonia and Latvia.
Lithuania became the first of the 15 Soviet republics to declare its independence in March 1990.