Here's a couple explanations I found...
sideways cross necklace is inspired by jewelry worn by celebs like Jessica Biel and Michelle Williams. (possible OOPS or artistic license?)
One peron's idea... the sideways cross should mean humanity, earth, or Imbetween heaven and hell.
On graves it is a reference to the hope of the resurrection. The cross has been taken down because Jesus has completed the work of his sacrifice. ("It is done")
There's also some speculation about it being a reference to something that's sure to land this thread in the shed, so I'll just leave it at that.
"I do not seek to understand that I may believe, but I believe that I may understand: for this I also believe, that unless I believe I will not understand." --Anselm of Canterbury
For consider Him who endured such hostility from sinners against Himself, lest you become weary and discouraged in your souls. You have not yet resisted to bloodshed, striving against sin.