Webhdfs seeks involve closing the current connection, and reissuing a new open request with the new offset. The RPC layer caches connections so the DN keeps a lingering connection open to the NN. Connection caching is in part based on UGI. Although the client used the same token for the new offset request, the UGI is different which forces the DN to open another unnecessary connection to the NN.
A job that performs many seeks will easily crash the NN due to fd exhaustion.