BroJoeK's theory is one answer. Another is that Confederate spies found out that Lincoln had sent a flotilla of ships loaded with men and war material to reinforce Sumter, in direct contradiction to the message he had sent to the Confederates informing them he would not send any war material.
April 4, 1861
To: Lieut. Col. H.L. Scott, Aide de CampThis will be handed to you by Captain G.V. Fox, an ex-officer of the Navy. He is charged by authority here, with the command of an expedition (under cover of certain ships of war) whose object is, to reinforce Fort Sumter.
To embark with Captain Fox, you will cause a detachment of recruits, say about 200, to be immediately organized at fort Columbus, with competent number of officers, arms, ammunition, and subsistence, with other necessaries needed for the augmented garrison at Fort Sumter.
Signed: Winfield Scott
April 1, 1861
To Captain H.A. Adams
Commanding Naval Forces off PensacolaHerewith I send you a copy of an order received by me last night. You will see by it that I am directed to land my command at the earliest opportunity. I have therefore to request that you will place at my disposal such boats and other means as will enable me to carry into effect the enclosed order.
Signed: I. Vogdes, Capt. 1st Artly. Comdg.
Captain Adams REFUSED TO OBEY THE ORDER and reported to the Secretary of the Navy as follows:
It would be considered not only a declaration but an act of war; and would be resisted to the utmost.
Both sides are faithfully observing the agreement (armistice) entered into by the United States Government and Mr. Mallory and Colonel Chase, which binds us not to reinforce Fort Pickens unless it shall be attacked or threatened. It binds them not to attack it unless we should attempt to reinforce it.
Presumably the agreement applied to Ft. Sumter as well. If it became apparent to the Confederates that Lincoln was deliberately going to land a force and reinforce Sumter, then they knew there was no negotiating with him, and might as well take the fort before his forces could land and reinforce it.
If the Confederates knew Lincoln was intent on starting a war no matter what they did, they might as well land the first Punch. That is what I would have done.
Trouble is, they didn't think through the politics of it. They would have been better off to let Lincoln be seen as the aggressor, but it is unnatural for men to let someone deliberately attack them and do nothing about it.
To accept your theory, you would have to believe that even if Sumter had been reinforced, rather than simply resupplied, that it would have posed some grave danger to the City of Charleston. Sumter could not possibly hold enough troops to threaten the overwhelming number of Confederate troops surrounding it.
It could not have possibly done so. It was only a symbolic presence of US authority, which after four months of stand off, Davis could not allow to continue if he were to move the upper South to secession.
IMHO, he made a foolish decision and was even told that by his Sec of State, but he did get the Upper South to join him.