The sparks you describe
The sparks you describe sound like airborne dust that is floating into the kiln when you open it. The dust hits the 1500F air and ignite.
The faster you cool the kiln 1) the harder it is in the kiln and the elements. You don’t need to crash cool – you can just let the kiln naturally cool. If you are going to open it, I recommend you only open it a few inches. Not sure why bubbles would grow as the result of rapid cooling.
What kind of controls do you have on the kiln?
Helios Kiln Glass Studio