Question

Why did the USSR attack South Korea?

Answer 1

It is not really the USSR that attacked (at least directly) South Korea:

Explanation:

The attack was conducted, in 1950, initially by North Korea (certainly supported and supplied by the USSR and China).
In October of the same year Chines forces invaded the North to counter the US/UN forces and repel them towards the south and reestablish the communist rule in the north.
It is certain that the army of North Korea was equipped and trained by the USSR but they were not involved directly in the fighting.

[North Korean tankers with Soviet made T-35 tanks]]