As for a singular event, there is none.

The reason(s) behind the attack on the Soviet Union are to be sought and found in the Nazis' ideological program. The USSR as the perceived home of Judeo-Bolshevism was seen as a threat that had to be dealt with under any circumstances. As soon as the Nazis had started the war, an attack on the Soviet Union was to some degree inevitable.

The two central core tenants of Nazi ideology were race and space, meaning the imagined fight of the German vs. the Jewish race and the idea that Germany had to have Lebensraum in the East as it had been their historic right to have. All this involved fighting and beating the Soviet Union. Additionally, Bolshevism was seen as one of the main enemies of Nazis as a tool of the Jews to attack the German Volk.

The planning for this attack had started in December 1940 but had been discussed long before that and to a certain extent was always in the cards as a long/mid term plan. The Hitler-Stalin pact was seen as a convenient way to delay the whole thing until the time was right.

Sources:

• Evan's Third Reich Trilogy.

• Lonra Weddington: Hitler's Crusade

• Bartov, Omer (2001). The Eastern Front, 1941–45: German Troops and the Barbarisation of Warfare. London: Palgrave Macmillan.