Some Nexus 6 Owners Reports Defective Back Cover Unglued
A few Nexus 6 devices suffering a build-quality failure where either the back-plate loses adhesion to the chassis of the device, or the back-plate is pushed off due to an expanding battery, depending on whose account you read.
Where one issue could be construed as a simple manufacturing deficiency, the other issue over an expanding battery is possibly causing for a bit more concern. One Nexus 6 user took to Twitter to contact Motorola Support over having not one, but also a second replacement Nexus 6 lose adhesion on the back plate. Another Nexus 6 user claimed on Reddit that the issue of his device disassembling itself was due to an expanding battery.
Motorola acknowledges that it has seen "a few" bad devices in the mix and is prompting customers to contact the company's customer care team. You can also try bringing the device back to the point of purchase as having something like this happen (without dropping it) is a manufacturing defect.