"Why do you think the federal debt exploded?"

Structural Operating/ Primary deficits.

Even at low interest rates, if you run structural Primary deficits, your debt burden will increase.

But you aren't entirely wrong - the return to Primary surplus by Mulroney was only enough to "hold the line" in terms of keeping the deficit down, given the level of debt he inherited, and the growing real interest rate of the period -  real interest rates on Canadian debt peak with the '91 - '92 recession