I understand there were Creole pressures, but not much else. Were there economic/familial/political reasons?
Before declaring independence in September 1822, Brazil was elevated from a colony to a kingdom in 1815, becoming a constituent of the United Kingdom of Portugal, Brazil and the Algarves. This legitimized the Portuguese royal family's presence in Rio de Janeiro after Napoleonic forces were defeated (the war being the reason why they fled from Europe in 1808).
This increased power was well enjoyed by the Brazilian aristocracy*, giving equal status to both Lusophone sides of the Atlantic.
Most of the Portuguese court was forced to return after a liberal revolution broke in Portugal in 1820, which also restricted the King's powers through the new Parliament, the Cortes.
João VI left his heir apparent Pedro and appointed him Regent of Brazil.
After the King's return, the Cortes sought to reduce Brazil again to a colony, which could also mean economical exclusivity - Brazil would only export products to Portugal again. This wasn't much acceptable to Brazilian landowners.
And being more familiarized with an absolutist regime (having moved to Brazil at the age of 10), Dom Pedro and the Brazilian leaders had in the new Portuguese parliament a common foe.
In late December 1821 the prince was called to return to Portugal, ending his regency. After local leaders pleaded, Pedro declared in January 1822 he would stay in Brazil, defying Portuguese orders.
His increased popularity and the Cortes persistence to have him back in Lisbon would drive him to declare independence in September.
I wouldn't count much on family issues (these would happen later in Portugal as Pedro's brother Miguel would usurp the Portuguese throne and result in the Portuguese Civil War), though there are registers of D. João writing a letter to his son "to put the crown on his own head, before some adventurous man does".