A number of iPhone 6 and 6 Plus owners have been reporting an issue with the device that causes it to crash and remain stuck in a boot loop following a restart



From MacRumors:

The issue appears to primarily affect 128 GB models, particularly those with larger app libraries, and some suggest it may be a hardware problem associated with the increased storage available on that device. While the exact cause of the issues has not been determined, speculation by industry contacts speaking to BusinessKorea (via G for Games) suggests the problem may lie in the controller for the TLC (triple-level cell) NAND flash included in the device. This issue reportedly is new to the iPhone as Apple previously has typically used MLC (multi-level cell) NAND flash in its iPhone units. The only recourse for owners experiencing this boot loop is to bring their phone back to Apple and request a replacement.

  Owners of 128 GB iPhone 6 and 6 Plus Models Reporting Crashing and Boot Loop Issues