What could cause a crowned tooth with a root canal from 2 years ago to suddenly become sensitive to sweets?

Most likely the sweet sensitivity is coming from an adjacent tooth. Root canal treated teeth do not have the ability to get sweet sensitive. Sounds like a cavity. See your dentist before it worsens.

This unfortunately can't be possible , it has to be an adjacent tooth.  I would seek care from a dentist in your area. Sounds like you may have a cavity in a tooth near where the root canal was done . 

HI:This is not normal.  It’s best to have a dentist evaluate the tooth and take an X-ray to see if there is a decay on the teeth adjacent to the root canal tooth.Best,Dr. Maddahi