Pollination issues most likely. Drought and heat stress can interfere with pollination. Also, having too few plants, or plants in a single row, can interfere with proper pollination.
Kim

Here is a link that might be useful: My Garden

In almost any stand of corn with more than a few stalks, you can see this sort of thing happening. Its just a throwback to teosinte, corn's wild ancestor from Central America. Teosinte doesn't grow ears, the kernels form right on the tassels. Ears are the result of selective breeding by humans, not the work of nature.

It has nothing to do with fertilizing or weather (at least not by itself).

Yeah, it's a genetic trait. It is a throwback trait to when corn (or a corn ancestor) had female and male flowers in the same spot. (not necessarily teosinte though as teosinte also has seperate male and female flowers)

If you don't like it like that, then dont save seeds from that plant, and/or choose a different variety next year. If you do like that trait, you probably could breed that trait to occur more often.

Here is a link that might be useful: Corn with cob on top

I agree, its normal. I had a few suckers produce open cobs right under the tassel. I pulled them off so it wouldnt attract any more bugs than were already there.

I also found a few normal looking ears with 2 cobs inside :)

I wonder if commercial farmers have people who sort these anomalies out? Ive never seen them in stores like this.