Why Does My Truck Battery Die When The Door Is Open?

Well I Just Bought My First Vehicle. Its a 1995 Chevy S10. 4cyl 2.2liter. Its Been Plagued By Issues after issues. The Battery Was Completly Dead. It wouldnt Hold a Charge. So I bought a Junk yard Battery. its a interstate Magnatron II. So I brought it home and put it in the truck and it started up without issue. So yesterday i was working on the interior and i had the drive side door open for 15- 20 min. The Battery Died Within that time frame. What Would Cuase that? Also I had it parked in the garage. Which has a coment Floor. Could it be grounding out?