Quite a big one.
Let them hold their referendum.
It could either go there way, or it won’t
They’re scared because they know the Catalans will win it.
Letting them hold the referendum when it has been deemed illegal by the central government would in and of itself accept that to some degree Catalan is independent, or even that it has a right to be independent at all even if the vote did succeed. If it’s a foregone conclusion that you would escalate to civil war over succession, then why respect the formalities of a vote that would grant the succession movement a formal popular mandate?
In the United States there would likely be a similar reaction. The civil war here settled the question of whether or not a state could leave the union. The constitutional government isn’t something any particular state can vote it’s way out of, and any actual attempt to succeed is by precedent considered an act of treason.
Spain has no reason to threaten the security of its state, so good.