As mentioned in SevenSidedDie’s answer, this seems to have entered the gaming lexicon after appearing in a Something Positive comic.
I was the DM in the D&D game Randy was playing when he started S*P (other than Mike and PeeJee, the players in this comic and the next few are based on our group). If I remember correctly one of the other players mentioned “Rocks fall, everyone dies” during a moment of levity. He said that he had a DM that he used to play with that used it when he got frustrated with the players.
It was a long time ago, and I may be conflating the memory with something else, but I believe the original form as related at our table involved the DM picking up all the dice he had and dropping them on the table as if they were damage dice, and then pronouncing “Rocks fall, everyone dies”.
So the expression pre-dates Randy’s use, but he certainly did popularize it.