The Red Army thought that they were liberators. They assumed that the workers of Finland would throw down their arms and welcome them with open arms. However once Soviet bombers started bombing working class neighbourhoods, that illusion was quickly shattered.

It is also certain that the Finnish workers knew from before the war what waited for them should the Red army win. Some leading Finnish communists in Moscow were murdered during the purges and that knowledge further bolstered FInnish resolve to fight.
The Soviets still believed that they would be welcomed and didnt prepare for a long fight, even sending in brass bands with some divisions to celebrate their victory in Senaatintori!

Hyvää Suomi!