Its by design, unlike the Nexus 5, the 5X doesn't have adhesive strips to hold the back plate to the frame, just clips. A lot of people complained about those strips with the old N5 as it makes repairs difficult, so I'm guessing that's why LG removed them. This makes repairing the phone and removing the back plate much easier. It's also the reason why the back of the
phone creaks when pressed.
I noticed this with my Nexus 5X and did some digging, a user explains it well in the nexus product forums,(i would post the link, but being a new user i can't

). This is also shown in the ifixit treardown of the Nexus 5X.