The way I understand it, it is because Fort Sumter was in Confederate territory. After the Southern States left the Union, they asked federal troops to pack up and leave. The Union refused. So the Confederacy, which considered itself a new sovereign nation, was in the position of having military bases from a neighbouring country sprinkled across it's territory.

So the North's refusal to pull out its troops from the South could be considered the hostile act, while the South's attacking these bases on their territory a quite natural response to it.