What would cause a new air compressor to blow back oil from the dip stick hole?

first ..with engine switched off ..check sump oil level ...if it ok ...you have crankcase compression ..this is a serious issue especially if its a new unit ...its when the engine compression is going behind the rings ..it goes down into sump ..the next revolution pushes even more down ..this pushes upward the first lot of wasted compression ..this mix will have fuel in it ..so your actual oil levels may rise ..but its not pure oil 

Maybe it's over-serviced, but you shouldn't pull the dipstick while the compressor is running.

the rings not sealing  or a outlet valve jammed assuming its electric driven by belts

Normal if you're checking it while running. If this is a gas powered compressor, which dipstick?