In Sun Tzu terms tech is like fighting in a marsh, where you have to keep moving or sink.
It wasn't that long that MongoDb was the disruptive technology. Now a blizzard of noSql db's are disrupting the disrupters. Google's Sergio Bossa the reasoning for moving from MongoDB to Cassandra:
the JVM, and we have lots of in-house experience on it.
It scales in terms of processing and storage capacity.
Its column-based data model gives us some advanced capabilities
Its tunable consistency levels provide greater control over high availability and consistency requirements.
As regards what made them look into a different solution:
We need higher resiliency to faults: MongoDB provides replica sets, but we’re experiencing lots of problems with replication lags and during replica synchronization.
For scalability: MongoDB global lock and huge memory requirements aren't already going to cope well with our growing data set.
For good measure he is not a big fan of Scala either http://thegeektalk.com/interviews/sergio-bossa/