Check your detailed summary.

Click 2018 Tax refund on the left just under the refund.

Click "View Detailed tax summary"

Scroll all the way down to "Payments and credits"

You may have a CPP or EI over payment. 

Climate action incentive at line 449 is a refundable credit if you lived in a qualifying province. 

You may have the refundable medical or Working income tax benefit. 

In any case, there are many reasons you could be getting more back than what you paid in.