At last: 'My infinite patience was exhausted.

Leaving my books and writings, after the service of thirteen years, persisted in with luckless constancy, I wandered forth like a new Bias, and betook myself to Mantua, where I met with the same treatment as at Ferrara.' This account sufficiently betrays the diseased state of Tasso's mind.
Being really deranged, yet still possessed of all his literary faculties, he affected that his eccentricity was feigned.

The duke had formed a firm opinion of his madness; and he chose to flatter this whim.
Yet when he arrived at Ferrara he forgot the strict conditions upon which Alfonso sanctioned his return, began to indulge in dreams of greatness, and refused the life of careless ease which formed part of the programme for his restoration to health.

In these circumstances he became the laughing-stock of his detractors; and it is not impossible that Alfonso, convinced of his insanity, treated him like a Court-fool.
Then he burst out into menaces and mutterings of anger.

Having made himself wholly intolerable, his papers were sequestrated, very likely under the impression that he might destroy them or escape with them into some quarter where they would be used against the interests of his patron.