This question was posed to me in examination.
This intriguing question comes from Deep Drawing topic in section Mechanical Working of Metals of Casting-Forming-Welding II
The best I can explain: In deep drawing process, the metal at the bottom of the punch is basically subjected to circumferential tensile stresses and longitudinal tensile stresses also. The punching force applied on the bottom of the cup is transferred to the side of the metal cup. And a narrow ring is present just above the bottom of the metal cup, which undergoes plane strain condition.