Trudeau and the Liberals were in power at the start of the 1984/85 fiscal year and wrote the budget for the year.  Mulroney won the election and took office right at the very end of that fiscal year.

Yes, I know the CBC incorrectly attributes this to Mulroney, but you really need to look at who wrote the budget to determine who is most responsible for the deficit, and in the case of the 1984/85 fiscal year that was Pierre Trudeau and the Liberals.

I specifically mention that year because it is the largest post-WWII deficit in Canadian history in terms of percent of GDP at 8.1%.  By contrast the $55.6B deficit for the 2009/10 fiscal year, while the largest in nominal dollars, was only 3.5% of GDP.