And traditionally, cabinet ministers would resign and stand for by-election upon appointment to cabinet, so this concept isn't totally unprecedented.
Not quite exact. They didn't do it by tradition, they did it as a matter of law. It was called Independence of Parliament Act, which was repealed in the early 1970's under Pearson for the 'official reason" that ministers were always re-elected so it was a waste. That is about the same time they started to introduce nomination of party leaders by American style conventions as opposed to selection by caucus.
Guess who the Parliament's independence was supposed to be protected from: the executive branch. In those days, MP's knew that their role, regardless of party affiliation, was to represent their constituencies in scrutinizing the executive and keeping it from excesses by control of the budget in particular. Therefore, once elected to represent the people in watching the government, it made sense, if selected for ministerial functions, that you would have to be re-elected by the same people to show they were OK with the change of function.
Moreover, in those days, party affiliation was just that, an affiliation. That is you presented yourself to the public as leaning towards a given party's view of the world, but not exclusively and party's were not electoral machine requiring obedience in those days. In a given riding, at the time of elections, you could have two or more candidates who claimed affiliation to the same party standing for Parliament, so character and popularity of the candidate mattered.
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