armor stand dropped down a few level on it's own

I was building a golem farm and placed a couple armor stands next to one of the pens so I could place the chain mail armor on it.

After a while (including loggin in/out) I went back and found the stand on the level below. It was occupuing the same block as a candle.

They were sitting on two stone slabs (top and bottom so that they made a full slab) and the block below (about 4-5 block below) were also two stone slabs.

They were in the correct orientation and had the correct armor.

There were two stands side by side. One was at a slight angle. They were facing west.