Iphone 6 randomly shutdown in standby mode

My Iphone 6 16gb randomly shutdown in standby mode, i only can turn it on by hold POWER + HOME. here is the log :

{"name":"Reset count","bug_type":"115","os_version":"iPhone OS 8.1 (12B411)"}

Incident Identifier: F0B79DBE-0BE1-431A-813E-70E7E2C17942

CrashReporter Key: 7305c452a84bf7ce3738c4bedf794c8e76fc4c58

Date: 2014-11-24 09:17:58 +0700

Reset count: 1

Boot failure count: 2

Boot faults: wdog btn_rst

Boot stage: 32

Boot app: 2681261667

Is this a hardware issue or software issue? thanks