Had a similar problem. FIRST check to see if the battery hold down bracket is metal. If it is and the "rubber/plastic" protective coating is corroded off you may have a problem. The metal hold down will act as a "short" to ground due to mositure on top of the battery.
Look at it this way. If the top of your battery is wet usually the wettness is water and battery acid. Battery acid is a conductor. If you applay a wire (or metal unprotected) hold down to the top of the battery then you can short the battery. It won't be a "direct" sudden short like a cable touching the body but rather a slow short that drains the battery.
Eric,
Burke, VA