Why did pontiac begin attacking british forts and settlements in the west?

When France was expelled from Canada in 1763, many indigenous groups (13 tribes) rebelled against the new British occupiers, unhappy with their treatment and the presence of settlers eager to settle on their lands around the Great Lakes, in the current border between Canada and the United States.  

The leaders of the insurrection were Pontiac (Ottawa), Guyasuta, (Seneca) and Kaske, (Shawnee), influenced by the prophet Neolin, who proposed to return to the indigenous life expelling the Europeans and all their customs.

At first, the rebel action was based on the destruction of the forts of the area and the extermination of all its inhabitants. At Fort Pitt, after looting twelve military places, the Delawareans were presented with blankets that had been in contact with the sick and the smallpox, with the idea of infecting them, although without results. Shortly after, the objective was the caravans of supplies and, later, the harassment of the settlers that were being installed in Virginia, Maryland and Pennsylvania. The violence of these attacks produced the reaction of the settlers, who organized systematic massacres of Indians, until in 1764, Benjamin Franklin achieved a precarious peace supported by regular troops. In parallel, it was again offered, as in other wars, a reward for killing an Indian, whether man, woman or child. The British later tried conciliation through treaties. Two years later a formal peace treaty was signed in the Colony of New York.

answer; negative punishment;