Dead battery troubleshooting advice needed

car (08 tundra) started strong in the morning, the usual 10 mile commute to work. parked for 8 hours, and battery dropped to 7.5v and no longer start. 2.5 year old Interstate battery. I didn't leave the headlight on or anything like that. I measured the parasitic draw while I changed the battery and it's about 6 to 7 mA. what could be the cause of the dead battery?