I've heard that Virginia's legislature was originally marginally unionist but then voted for secession after Lincoln called for troops to put down the rebellion after the attack on Fort Sumter. Why did this increase the calls for joining the Confederacy when the South was the aggressor?
Because they felt the Southern Confederacy was well within its rights to remove foreign troops from its territory.
Many saw secession as a legal right of State Sovereignty under the 10th amendment. While those states didn't agree with secession at first, they saw the Confederate States as a legitimate political entity and US occupation of southern forts as illegal as no treaty existed for their occupation.
Fort Sumter is located in the harbor of Charleston South Carolina, the heart of the Confederate States. Federal forts were being surrendered and abandoned all across the south. So the assumption was that Fort Sumter would be the same.
This refusal to surrender the fort was seen as the belligerent act. Forcing its surrender was unfortunate but within a Sovereign Nations rights. The North's call for 75,000 volunteers was considered the belligerent act as the independent Confederacy's seizing of its forts was considered the normal and natural thing to do.
An imperfect analogy would be if a family rented out a guest house in the backyard to someone. The rental lease is up and the family no longer wants a renter so they don't renew the lease and give a notice of eviction.
But instead of leaving the guest house, the renter refuses to leave. So the family calls the police. After a short standoff the police drag the guy out. Unfortunate, but necessary.
But instead of the renter moving on, he and some of his buddies come back.
That's the Southern point of view. They felt the federal government was committing illegal actions against an independent nation