Degree of Freedom for a beam structure

I created a beam structure that mainly consists of the channels and boxes. The piping is running and supported inside the channels and boxes (see the images attached). The structure is attached to a side wall (not shown in the images) at two locations  A1 and A2 with the pin connections (free rotation about the X axis). This structure is anchored to the floor (or deck) at multiple locations with thermal anchor movements (TAMs) and seismic anchor movements (SAMs). The horizontal beams are also pin-connected to the vertical posts.

When I ran the analysis, Autopipe warned the system was unstable and the analysis was aborted.  In order to make it run I had to change the connection at point A2 as fixed. Can you please help me to figure out why the pin connection at point A is a problem ? In theory  the structure is well restrained and the pin connection should not cause a free body motion. By the way, the structure was analyzed in ANSYS with no restraint problem.

Thanks,

G Song

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