The story here is simple: hit a crazy ☞ bug in Cassandra (remember all these tools are really young), they needed their data before a fix was available (keep in mind that some are using these NoSQL solutions in production), migration to MongoDB (while in production, you’ll do whatever it takes to minimize downtime):
At some point we started to have some stability issues with Cassandra. All nodes would go into an infinite loop, running GC and trying to compact the data files – occasionally falling off the cluster. We were unable to solve the problem, except that restarting and then compacting a node usually settled it down for a while. Other people had reported similar problems. Last couple of weeks our Cassandra nodes always ate all the resources they were given, slowing down Flowdock.
Anyways, I think there’s a lesson here: all NoSQL databases should start providing a data export tool.