Why is ANSYS sometimes adding DOF constraints automatically when using Sparse solver and small deflection?

NOTE: ANSYS issues the following warning in the output:

*** WARNING *** CP = 4.969 TIME= 08:45:58
Small equation solver pivot term encountered at UY DOF of node 2.
ANSYS automatically constrained this DOF. Check for an insufficiently
constrained model.

With PCG solver, ANSYS crashes with:

! *** FATAL *** CP = 2.297 TIME= 10:
! Negative or Zero pivot; Pivot Value = 0.000000e+000.

We add the constraint to prevent rigid body motion. We are constraining the DOF that gave us the small pivot term (UY DOF of node 2 in this case). It is an appropriate stabilizing constraint in most small deflection applications. However, it might not be appropriate in all cases, if the user did not intend to constrain the model in this way. That is why we issue a warning.

If the additional DOF constraint is deemed inappropriate, try turning on large deflection (NLGEOM,ON). ANSYS will have additional terms in the stiffness matrix that allow it to get around the pivot problems.

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