Diff oil out the vent, why?

There are only a hand full of reasons for oil leakage at the breather.

No particular order

1. Too much oil
2. Too light of an oil?
3. Slinging from the ring gear
4. No sealant on the threads.
5. MAYBE... If it is clogged and the differential builds pressure from heat and then valve opens and a woosh of oil air comes rushing out