I don't usually put scrap foods on top so I am curious why african night crawlers are always on top of my bin? Is it because there's a lack of oxygen on the bottom part of my bin?
Not necessarily - if you mean they're in the top layer of the contents, rather than separate from them and hiding in the lid of the container, they tend to consume stuff near the top of the pile for preference, operating primarily in the top layers - more info in the link below, including good things to include for ANC specifically
Based on Bamboo's answer link, I would like to quote the information that helps me
Typical of all composting, or vermicomposting, worms ANCs come up to the surface of their bedding to eat decomposing matter. So they thrive near the surface layer of top soil or bedding. African night crawlers literally gobble up decaying matter. Watching a few hundred ANCs feed on some fruit or vegetable scraps is an amazing thing, we have simply not seen any composting worm pounce on food in this way.
It seems that it's the habit of ANC's to be near the surface layer top soil.