Hi, thank you for using Intuit turbotax Live Community
The Canada Revenue Agency (CRA) can reassess a tax return for many reasons. The notice of reassessment will usually have an explanation section that will detail the changes made. It is hard to determine the cause of the reassessment without reviewing the notice of reassessment.
Look at the explanation and see if that helps. You can also contact CRA at 1-800-959-8281 for an explanation.
Reassessments are relatively common and could be due to additional slips, calculation errors, etc.
If you need further assistance you can contact us. Contact information can be found at:
https://turbotax.community.intuit.ca/community/product-information/help/i-want-to-talk-to-someone-ho...
Hope this helps
