Food for thought from the concluding paragraphs of a study of provincial coalition governments by G.P. Marchildon, a University of Regina political scientist best known for his long association with Roy Romanow:
Once a coalition is created, its longevity is determined by a number of factors... The less partisan and the more willing party leaders are to share power with the members of another party, the longer the coalition will survive. Based on the provincial case studies, it is obvious that changes in party leadership can have a dramatic impact on the coalition and those party leaders who create the original coalitions are in the best position to maintain them, in large part because they have invested their political reputations in the coalition and its survival. In addition, the more charismatic and persuasive a coalition premier—John Bracken for example—the less likely it is that a powerful anti-coalition faction within a party will emerge under another charismatic leader capable of challenging the coalition premier.
...Coalitions are difficult to maintain with populist parties because they often place significant constraints on the decision-making of their elected representatives, particularly those in cabinet. Since the Westminster model tends to push the process of coalition accommodation into the cabinet room, ministers who cannot make compromise decisions without full consultation and approval of their party executives face a difficult term of office. In the provincial studies above, they often become estranged from their non-elected party members. The Ontario UFO-ILP coalition in 1919-23 is a striking illustration of the phenomenon.
Why do coalitions end? Based on the studies above, the unravelling of coalitions often begins with a change in premiership and party leadership. New leaders are almost by definition less committed to the coalition. In other words, death, illness, or retirement of the party leaders originally responsible for the creation of the coalition can be lethal to its continuation. The most unstable coalitions can be those where a party leader does not consult with, or obtain the majority consent of, the party members for the establishment of the coalition in the first place since this often precipitates a break between the party leader and the party executive.