You are referring to the events of the Sicilian Vespers in 1282. The intervention of the Aragonese was not as well-coordinated as you might expect. Their fleet under Peter III was in the process of intervening militarily in North Africa in order to cement control over trading interests in the southern Mediterranean and thus happened to be in the close vicinity.

The amount of direct planning that went into the uprising on the part of the Sicilian rebels is unclear but the outbreak of violence does seem to have triggered by the molestation of Sicilian women by French troops at the Church of Santo Spirito near Palermo. (Abulafia, Frederick II: A Medieval Emperor, 424) There is no question that the Byzantines played a significant role as well as they had attempted to promote disaffected Sicilians interested in plotting an insurrection. Charles had intended on expanding his domains in the eastern Mediterranean violating what the Byzantine's viewed as their rightful territory, and they were committed to undermining his preparations and internal politics wherever possible. Reflecting upon the aftermath the Byzantine Emperor Michael Palaeologos contended, "Should I dare to claim that God brought their freedom and that he did it through me, I should only be telling the truth." ('imperatoris Michaelis Palaeologi De Vita Sua,' ed. H. Gregioire, 8, xxix-xxx (1919-60)).

The populace who had despised the French and their oppressive treatment for years rose up in the ensuing days and slaughtered thousands of French soldiers and civilians. There was greater coordination in the days following the massacre with representatives from each district assembling to formulate plans for a future government. The decision to form a commune and petition the pope did not however yield the desired results as he flatly rejected their requests and called for a new French invasion. The rising did not occur everywhere at once, and word had to travel across Sicily over more than a week for men to mobilize in strength.

Peter's fleet was thus perfectly placed to intervene as the crisis unfolded, and he had a rightful claim through his wife who was the daughter of the former king Manfed who had been deposed by Charles of Anjou. However, the important point to remember is that the Sicilians at first did not favour Aragonese rule either. It was only due to the rejection of their demands to form a commune by Pope Martin and his demand for the restoration of French rule that they turned pragmatically to Peter as a comparatively tolerable solution. He promised them trading rights and privileges that they had historically enjoyed in the twelfth century and was duly crowned in September of that year. Peter was excommunicated and hostilities were renewed leading to the War of the Sicilian Vespers which spilled into the fourteenth century.

Abulafia, Frederick II: A Medieval Emperor

Runciman, The Sicilian Vespers

'Imperatoris Michaelis Palaeologi De Vita Sua,' ed. H. Gregioire, 8, xxix-xxx (1919-60)