One of the niceties of open-source development is that once in a while people will make valuable contributions. By valuable contribution, I mean code that shows intimate knowledge and understanding of the software. In my estimation, in the log4j project, we got one valuable contribution every semester, roughly speaking. In my astonishment, in the logback project, log4j's successor, we get a valuable contribution every two weeks, again roughly speaking.
I really can't explain it. Log4j has more users than logback. So it is definitely not the size of the user-base. You have two similar projects. They have similar code bases and similar architecture. Log4j is much better known, yet it is logback that gets the more frequent contributions.
Logback is newer, and has not reached 1.0 status, so perhaps developers feel more confident that their input will be acted upon. Also, logback is functionally richer so it may be more attractive to contributors. It may also be just luck, although given the nature of independent random processes I doubt it.