Officially, all elected representatives are accountable to the Crown - the King or Queen of Canada. Within provinces in Canada, the Monarch is represented by the Lieutenant Governor. When the leader of the majority party of Manitoba, called the Premier, needs to call an election, he or she informs the Lieutenant Governor, who then dissolves the current Assembly and calls the general election.
The Lieutenant Governor is appointed by the Governor General of Canada (on advice by the Prime Minister), and is largely considered an honourary post, though there is real Constitutional authority vested in the Lieutenant Governor. The Lieutenant Governor is responsible for ensuring that there is always a duly constituted government.