Well incredibly I jusr came across that story here
as Buddhist stories from the Dhammapada commentary:
Now at that time there were asuras dwelling in the world of the Thirty-three, and when
they learnt that new gods had been reborn there, they prepared celestial drink for them.
But Sakka gave orders to his retinue that no one should drink it. The asuras, however, drank freely and became intoxicated. Thereupon Sakka thought to himself. “Why should I share my kingdom with these deities?” Then, giving a sign to his retinue, he caused them to pick up the asuras by the heels and fling them into the great ocean. So the asuras fell headlong into the ocean. By the power of their merit there sprang up at the foot of Mount Sineru the palace of the asuras and the tree that is called pied trumpet-flower.
So Sakka and the new gods were young men in a previous life who performed merit. The footnote says that asuras and gods were perennial enemies but doesn't say if this situation was already established before this round of gods was born or not. Perhaps they were just karmically inclined to be hostile to one another.