Stalin had agreed at the Yalta conference that the Soviets would join against Japan "in two or three months after Germany has surrendered and the war in Europe is terminated." So its rough timeline was already laid out. He had initially been planning for the invasion to occur in mid-August 1945. It is not clear to me that this was done with any indication of when the first atomic bombs might be ready, though Stalin knew about the Manhattan Project and much of the information (but not all of it) on when the weapons might be available (he knew roughly at what stage the project was, and new a test of the bomb was pending, but didn't know more beyond that — frankly, neither did anyone else, pending results of the test, which happened while everyone was at Potsdam).

As it was, the Hiroshima bombing was earlier than Stalin had anticipated and he worried that the war would end very soon. So he accelerated his invasion plans and told his troops in the Far East to begin as soon as possible. Altogether they probably accelerated their attack by a week or two because of Hiroshima.

So it was partially bad luck — a coincidence of when the atomic bombs were ready — and partially not — the acceleration of the invasion to immediately follow Hiroshima.

Hasegawa's Racing the Enemy goes into all of this very well if you are interested in details.