Key Lookup isn't applied by default?
There is no Description column in your index but there is in your query. So, SQL Server has to get this column. There are two options for doing that:

non-clustered index seek + key lookup
clustered index scan

If your statistics make SQL Server think it will read a lot of data, SQL Server makes a choice between the non-clustered index seek + key lookup or clustered index scan, this called tipping point.
Because, if SQL Server has to read huge data, the clustered index scan can be more efficient rather than non-clustered index seek + key lookup.
There is a good blog post here.