I've been wondering the same thing... battery seems fine and none of my apps or keeping the device away, so I'm assuming it's the whole dt2w feature they have on it. Plus, they mention a "sleep mode" on the HTC website, so maybe they got their own thing or something...
Any info would be nice.
EDIT: Apparently, while charging the device, it will rarely go into deep sleep mode. After I took it off the charger, it was going into deep sleep exactly like it should.
HTC One GPE (m8)
ROM: Paranoid Android | Kernel: Stock | Recovery: TWRP 184.108.40.206
LG G Watch (dory)
ROM: Gohma | Recovery: TWRP 220.127.116.11