It looks like the glitch I discovered yesterday is actually caused when a bomb is still exploding when you end the level. If you do it right, you'll hear a brief explosion at the beginning of the next level.

My theory is that the ability for enemies to fire in this game is controlled by a single memory value, and when you use a bomb, this value is toggled off so that bombs give you extra safety, and the bomb's finishing explosion toggles the value back on. When you bomb at the very end of the level and it ends before the finishing explosion, the value isn't toggled back on and just stays off.

This would also explain why the game opens with a bomb being dropped (other than being a nice looking way to open the game); the programmers may have considered this a failsafe in case this value somehow defaulted to off after a reset or something. (Just my guess, but it would make sense.)