The cassandra issues were primarily ops failures, and secondarily an older version of Cassandra making it difficult to recover once it was overwhelmed. (Some of the resulting improvements in Cassandra are documented here: http://www.riptano.com/blog/whats-new-cassandra-065)
This could very well be, I remember one of the problems they were having is not having enough resources for cache or something similar. But I could be wrong I will ask them on twitter and see if they can comment here on it.
Cassandra looks to be working fine for reddit now: http://blog.reddit.com/2010/08/everything-went-better-than-e...