When you place a block, it's texture is usually rotated from the original texture to give off a variant appearance, and that is completely fine. But when you push or pull it by a piston, its rotation is updated so it doesn't have the same rotation as it was when it was placed by a player. This happens with all blocks that you can push and has rotation variants in their textures.

Steps to re-create:
Step 1:
Place a block, for example some stone.
Step 2:
Place a piston next to it and then power it*
Step 3:
Look closely at the block that was pushed and you can notice that it's pixels are changing every time it is pushed. (It is being rotated)
Step 4:
Tell other people (Optional)