When the CRA reassesses your tax return, they indicate why on your Notice of Reassessment.
First, review your Notice of Assessment and compare it to your filed return. Once you've done this, if you believe your reassessment was due to a TurboTax calculation error, here's how to initiate a review of your return under our 100% accurate calculations guarantee: https://turbotax.community.intuit.ca/replies/2647678
