Couchbase data have been lost

hi everyone.
recently I faced a problem in Couchbase, we had more than 2.5 million documents , but we suddenly recognized that 1 and half million data are lost…!!!
we don’t know what’s going on and what happened which this issue happened, but it affected our product and it was so bad for us.
Fortunately we had back up of data every 24 hours, then had to restore our last 24 hours back up, and we started working again with those data, although the last hours data have been lost.
extra info:
-Couchbase is a cluster of 3 servers,
-we had more than 260000 channels and 260000 users, and as it goes on it grows more .
-2 servers of sync gateway .
It would be appreciating if someone can help me, because I don’t know really why it happened!!!

I think you start failover process in badly manner

@socketman2016 , actually one node failed and we started rebalancing, but also we had 1 replica, so I suppposed that we won’t loose any data, but we did.
1 question, what do you mean in badly manner?
how it occurs and why?
what should we do for not facing the same problem again?