Changing polar alignment...Ground shifting?

Tom Blahovici

I thought I would ask the experts here about my shifting polar alignment.
5 years ago I rented a backhoe, and dug a hole 7 feet deep in my backyard.  I then poured a concrete pillar in a form 3'x3' by 6 feet deep.  On this I have a paramount pier.
All the earth was filled in and I have used this mostly over the winter. Last year all was fine.  This year I had about 1 minute of arc drift in each axis using pempro.
Recently I started noticing that my stars appeared trailed during long exposures and autoguiding was acting up.  PHD 2 told me my polar alignment was off by 4 degrees. . So today I checked my polar alignment with pempro and sure enough, 4-6 degrees off in drift.  I have put it back to within 1 minute.
I thought I was golden with such a pier. Recently we had a lot of rain and then the temperatures have been going between + 12C and -12C.  Could this be a factor?  How often do you check your alignment?

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