There are a few reasons why this could happen.

Your employer may not have taken enough takes off when they issued your paycheque.

If you changed provinces, then you are taxed based off of the province you are in at the end of the year, and that could cause you to owe more than what was taken off your payroll.

Finally, you may have income from sources other than employment, which may have not been taxed during the year, and so will be taxed on your income taxes.