I would like to request a year's unpaid personal leave from work in Ontario. Should I disclose the reason to my employer, and if so, how much detail is necessary? If I disclose for family reasons, are they legally allowed to ask for details? My collective agreement says that the leave can be granted at the discretion of the employer.

Based on the context included in your question, it sounds like you are employed in a unionized environment subject to a collective agreement. As such, the terms of your employment and the collective agreement would need to be reviewed and assessed to properly answer your question.

Your union representative/steward may be a helpful resource to consult in the circumstances.