Thank you for contacting eXtension. This is a good question! I have requested a copy of an article that I was unable to access online but it has not arrived yet. The answer might not even be in there, but if it is I will certainly let you know. In the meantime, you might want to contact Dr. Weil (<personal data hidden>) to ask.

Hello again! I asked Dr. Weil and got an answer right away:

Soil series are named after geographic features like towns or rivers. The Sassafras soil is named for the Sassafras River on the Eastern Shore of Maryland.

From Wikipedia: The Sassafras River is a tributary of the Chesapeake Bay on the Delmarva Peninsula in the United States. It is approximately 22 miles (35 km) long and starts in western New Castle County, Delaware, and along the boundary between Cecil County, Maryland on the north and Kent County, Maryland on the south.

Thanks again for this interesting question! The document arrived and I thought you might like to see it. The origin of the series name is addressed (highlighted in the attachment), and it adds a little context to the series-naming protocol.