Hi Dave,
This has been covered before but I'll try to explain. It's not a tradition as such. Most ships intended for demolition are sold through intermediaries often referred to as cash buyers. These cash buyers then hope to re-sell ships at a profit to shipbreaking companies, primarily on the Indian sub-Continent. When any ship is sold there is usually a name change. The cash buyer is therefore doing what any trading buyer would do and is renaming his ship - maybe in response to a contract clause. It just so happens that his voyage is often the last one to the breakers yard.
Brgds
Phil