On the FAQ it says:
Log4j has adopted a rather conservative approach by following the Code Conventions for the JavaTM Programming Language
However, if you check the examples in the Java Coding Conventions, you find that all the modifiers go on a single line before the function:
public void doSomethingElse(Object someParam);
If I'd have to guess, I'd say it makes the diff
easier to spot when committing patches and also maybe avoids long lines.