As we shall see in an up-coming tutorial, Cabinet or the Executive Council, is a decision-making body of the Executive Branch composed of appointed ministers of the Crown each responsible for a file (such as defence, finance, health) known as a portfolio. The Cabinet also includes the Prime Minister (or premier at the provincial level) as head of government. These ministers are appointed out of the caucus of MPs (or MLAs, MPPs, MNAs at the provincial level) from the party with the most seats in the Legislature by the Governor General (or Lieutenant Governor at the provincial level) on the advice of the Prime Minister.
So, essentially, they are appointed by the head of government and are therefore loyal to him or her. The number of Cabinet Ministers changes from government to government: it grows in membership or shrinks in membership on the will of the head of government.
Given this, it is the prerogative of the head of government to change the composition of the Cabinet and change who is in cabinet and in what position. From time to time, to either freshen the image of the government, to promote someone who has loyally served the government or demote someone who has fallen out of favour with the prime minister or premier, the head of government will shuffle the cabinet, literally move around, add or remove ministers. A cabinet shuffle may also come about because a minister has resigned or retired. Cabinet shuffles can come at any time but typically occur after an election or after the installation of a new head of government (as was the case with Kathleen Wynne’s government in Ontario).