It's almost certainly due to the spoke(s) being too loose (now or in the past). Unfortunately this means early failure of the spokes and even retensioning the wheel doesn't always solve it for long as several spokes may have been damaged and you'll gradually have more spokes break (basically, loose spokes flex more and therefore fatigue quicker.
The best solution is to rebuild the wheel and replace all the spokes. Otherwise, learn how to replace spokes one at a time (and carry some with you if you're likely to get stuck out in the middle of nowhere!)