It's the average figure so that doesn't explain it.
I was going to suggest that the population of the country may have dropped at the same time (as has happened in some Eastern European countries) but quite the reverse, so that's no explanation. Between 2014 and 2015 the population of Congo jumped by almost 8 million from 69 million to 77 million. The jump in population, combined with the drop in JW numbers, meant that the population to JW ratio climbed from 321 to 375. That's a huge change for a single year.
Maybe there was a mass defection or a rival religious group has been formed. It has been known to happen in Africa before.