The blue link on the QBI page has a long discussion of what is a trade or business (More Info about what's considered...).
Here is the most relevant paragraph:
"There is considerable uncertainty about whether a small-scale, long-term rental of a single property or small number of properties is a "trade or business." This applies to the relatively common situation where a taxpayer owns a single rental home or two, and has relatively stable tenants that generally stay for a year at a time or more. Numerous court cases exist where situations meeting this description have been considered a "trade or business," and others where similar situations have not been considered a "trade or business."
Basically, it's up to the taxpayer to decide if the 20% deduction is worth the risk that the activity is NOT a trade or business in the eyes of the IRS. TurboTax cannot make that decision for the taxpayer.