Why do I see TWO forms in my return - one "regular" and one "AMT"?

Hi Dear Community - 

Please help me understand:

In my past return, I see two forms (one regular and one AMT) for Schedule D, Depreciation and Form 8582 (Passive Acitivity Loss Limitations). Why? A bit confused. NO actual Form 6251 (AMT) is there.

Thank you.