"Pastor, we just feel led to leave" is occasionally the explanation - most often there's not one.
Then, with so many dwindling congregations looking for fresh meat, finding a new fellowship is easy - and with no questions asked. Of course sometimes God does lead us to new opportunities but we must be cautious because clever justifications for putting distance between ourselves and disappointment or responsibility is nothing new.
We find Moses in the desert, Jonah in the whale's belly, Elijah at Horeb, Elimelek in Moab, and Abram in Egypt - all who rationalized that it's time to go. However, nothing good took place in the lives of any of these men until they went right back to place they left and faced the threat, failure, fear, or disappointment. God gave each of them an opportunity to examine their motives more deeply, face their failures, or to trust Him more fully, but they ran.
This may explain a very sad and dangerous arrangement between congregations, which provides a safe-place that "protects" many Christians from the threat of personal growth. I use the word Christians because Paul not only calls them infants in Christ but he also refers to them as brethren (I Cor 3:1). Paul then reveals that the issue is not being an infant but rather remaining one... and today, the Church may have become an accomplice.
Solution... love. Loving each other enough to confront. One can leave a fellowship without explanation but joining a new one without transparency and resolution with the previous one should be impossible. We must avoid the temptation to fill seats and vacant positions with our promising new prospects until any necessary healing, repentance and forgiveness takes place. Remember, unlike the airline, churches never loose baggage... it may be delayed, but it always shows up.
Pastor Mike Snelgrove