Depends on whether you are getting oil or grease. If it is oil, then the inner seal is leaking. The outer seal might be leaking as well, but that won't matter so much as long as the inner is holding (and you don't put too much grease in the bearings).
In my case, the root cause turned out to be the retaining bolts going AWOL. Not sure what the story is, but there also weren't enough shims in place.