This question has a couple different facets to it, since Southerners saw themselves as victims in a handful of ways before the Civil War.

The most direct answer to your question is that the South saw Union occupation of federal forts to be in direct violation of their newly formed independent state. When South Carolina officially seceded from the Union on December 20, 1860, they made it clear to the federal government that all forts within South Carolinas borders were now property of South Carolina. This included Fort Sumter in Charleston Harbor. As more southern states left the Union, they all had similar views on federal forts within their borders. Most federal forts surrendered willingly by being abandoned after realizing the hopelessness of being deep in enemy territory. Fort Sumter in South Carolina and Fort Pickens off the coast of Pensacola, Florida were the only 2 federal holdouts. These forts created diplomatic issues for the Confederacy, because although attacking the forts would most likely bring on a war, leaving them unchallenged would weaken the Confederacy's claim of independence. In the end, they found that attacking the forts was the only way to assert their sovereignty since the President and the War Department refused to back down and give up the forts, even after attempted negotiations.

However, Southerners saw themselves as victims in other ways too. The attack by Northern abolitionist John Brown on the arsenal at Harpers Ferry, Virginia in late 1859 made Southerners very afraid of slave uprisings. While many newspapers and public figures in the North eulogized John Brown as a martyr after he was hung for his actions, the South despised him and was horrified at the Northern response. They saw John Brown's actions and the response by the North as a failure on the part of the federal government to protect them. Jefferson Davis himself said that if the South were invaded by "a thousand John Browns", the federal government would not be willing to protect the South.

Southerners also saw themselves as political victims. They saw threats to their way of life by the growing popularity of the abolitionist movements and the rise of the Republican party. Hot debates over whether or not Kansas should be a free or slave state were often personal attacks on the practice of Southern slavery and the plantation system. The admittance of multiple free states (Minnesota and Oregon), with no new slave states to balance them out in Congress made Southerners feel like their political influence was waning fast.

Sources:

Most of the Fort Sumter stuff I got from Shelby Foote's "Civil War Volume I: Fort Sumter to Perryville"

John Brown info is from "Midnight Rising: John Brown and the Raid That Sparked the Civil War" by Tony Horwitz

Another book I've referenced before for a lot of the prewar information is "The Caning: The Assault That Drove America to Civil War" by Stephen Puleo