Yes, we saw this same behavior.
A couple of months ago, we moved a large portion of our data out of Postgres and into Cassandra. The initial migration was done in a "distributed" manner: we had 600 (or 800, can't remember) processes reading from Postgres and writing to Cassandra in tight loops. This caused the exact behavior you described. We also did a read before a write.
After we got through the initial data migration, our normal workload is *much* less writes (and reads for that matter) such that our cluster can easily handle it, so we didn't investigate further.