Agree with those who put the blame on moisture. Actually, popcorn loses moisture as it ages. Usually "old" popcorn will have more "old maids" and may have an off flavor.
Moisture may come from too much corn in too small a place, too little venting, pulling the corn off the heat after popping but not taking the cover off right away and getting it out of the pan.
Also remember that butter is 20% water, the better you distribute the butter, the better you distribute that 20% of water into the popped kernels.
One good way to avoid that is to pop in a heavy pan, remove the popcorn immediately, the throw the butter into the hot pan. It will sizzle and melt and much of the water will leave as steam.
I too grew up on Sunday night popcorn. It was the only day we always had a large meal around 1 PM. At dinner it was all you can eat popcorn with a piece of homemade fudge plunked down in it. A little "Maverick" on TV, what more could a growing boy desire.