The T691 Alternative Minimum Tax is generated when a taxpayer claims certain tax deductions, exemptions or credits to reduce the tax that they owe to very low levels. The T691 does a different tax calculation than the regular income tax calculation, that allows fewer deductions, exemptions, and credits. If the amount of tax calculated on the T691 is greater than the amount of tax owing using the regular system, then the difference owing is payable as AMT for the year.

This TurboTax article explains what causes the AMT calculation to trigger: The Alternative Minimum Tax (AMT): How It Works and What You Need to Know