Answer:
They feared a domino effect (so that communism would lose its influence and it popularity would be contested)
Explanation:
The communist policies set by the Soviet Union were strict and imposed a harsh condition of almost non existent political opposition in the puppet states they influenced. Furthermore the states located in Eastern Europe created a "buffer" for an hypotethical war if the USSR was to be invaded. So they resolved to military occupy Hungary and restore the situation back to the conditions with the old regime.
The same situation ocurred with the Czech Republic and perhaps would most likely account for the History as one of the fiercest repression by that time.
