CassandraStorage appears to have threading issues along the lines of those described at http://pig.markmail.org/message/oz7oz2x2dwp66eoz due to the sharing of the UDFContext.
I believe the fix lies in implementing
public void setStoreFuncUDFContextSignature(String signature)
and then using that signature when getting the UDFContext.
From the Pig manual:
setStoreFunc!UDFContextSignature(): This method will be called by Pig both in the front end and back end to pass a unique signature to the Storer. The signature can be used to store into the UDFContext any information which the Storer needs to store between various method invocations in the front end and back end. The default implementation in StoreFunc has an empty body. This method will be called before other methods.