Two possible issues:

1) Parasitic battery drains; clocks, radios, computers, alarms, etc--these are always on.

2) The alternator may be going out. If the alternator is not putting out enough amps, then the battery is not getting charged fully.

I recently had both of these things happen to my 1996 Explorer. I rarely drive it, so I had it on a battery maintainer. That worked for awhile, then it stuttered and died when I turned off the key after a trip. Replaced the battery per AAA's recommendation and it worked for a few more trips then died due to a bad alternator. So, I had to replace that too. Now all seems fine again.

Take away: It is probably a good idea to have both the battery and alternator checked at the beginning of winter. [I actually had done this just before they both went out, but that may have been either the shop's fault or a fluke].