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.