No, it was Lincoln's attempts to re-supply and reinforce the fort which were the cause of it. Being fired upon was only the effect. But then the Yankees always played by two sets of rules. One for themselves, and a completely different set for everyone else. Now days we call this type the liberal left.
The confederate forces had been firing at anything flying the U.S. flag long before Lincoln's resupply effort. On at least two prior occasions they had tried to initiate hosilities by firing on unarmed ships. In the end, they came to the conclusion that the only way to get their war was to bombard the fort itself.