CPU usage among Numa nodes in sql server
The short answer is that the behavior you indicate is expected. A single thread by definition only exists on a single logical processor, so for a large single-thread operation, you will not see it equally spread across all nodes. It can't be since it only exists in one place. 

The longer answer is that SQL Server backup operations use one thread per data file for reading, and one thread per backup file being written to. The reader thread will create a set of buffers to store information which will subsequently be read from by the writing thread.

For instance if your database uses one file on the D:\ drive, and you are backing up to a single file on the E:\ drive, you will have one thread reading from D:\ and storing that information in buffer, and another thread reading from information in the buffers and writing it to the single file on the E:\ drive. 

If you back up your database to 4 files, you will have 5 threads: one reading from D:\ and putting information into the buffers, and four getting information from buffers and writing to the separate files on E:. 

In NUMA architecture, the design is that these related threads will exist on the same node to optimize memory access - after all, the four writer threads want information from the buffers that the associated reader thread is using. It only makes sense to keep them on the same node.

For further reading see this post by Shanky_621.