I'm guessing that it turned on in your pocket and was insulated by your hot hot body (yeah, I watch your videos) and over heated.
From the manual,
3. Do not leave discharged batteries in the ATAC Series Flashlight. This could lead to permanent damage of the flashlight.
4. Its normal for the flashlight's body aluminum body to get very warm during continual use.
Reading over the warranty suggests that since this happened during normal use they will fix or replace it. Give them a call at 1-888-511-4535. Good luck buddy!