The nationalism was always there. At first, it was strongly suppressed. Stalin in 1941 found it expedient to suppress it less (to try to motivate the population to fight Nazi invaders). Brezhnev in the 1970s found it expedient to suppress it even less (to try to motivate the population to not succumb to Western cultural influences). Then Gorbachev pulled out the control rods even further and it exploded.

Same reason the Warsaw pact and communism in all East/Central European countries collapsed at the same time. It could only survive as long as there was a credible threat of Soviet invasion.

Same applied to almost all of the national "republics". Most people were only willing along with it due to fear and hopelessness (since any type of civil disobedience would be violently suppressed).