Armor stand moves after using piston to push another block into it

I made a tablet PC design I saw in youtube using armor stand but something strange happened when I come back to my world a few days later.

The armor stand moved back a little bit making my tablet PC have a weird look. 

I think because I pushed another block into it using a piston?

This happened to my computer design too... The armor stand moved aside.

(Sorry I can't upload the screenshots. It doesn't upload...)