Because they are confused, and screw up often. Ok, you have PVC tubes, cork in the bottom and you're resting shells on top. The shells all have fuses, and wires. All shells have warnings about how they should only be handled by people with the knowledge of Moses, the insightful reflection of Sherlock Holmes, and the experience and steady hands of a surgeon, but you ignore that. You've never done this before, but it's ok.
Most shells are ready to be lowered into the guns by the wire.
You lower them down, take the wire and attack it to terminals on the rail. Don't cross them - an earlier shell leaving the mortar might pull out the wires of a later shell, which would cause it to fail to work. It reminds you of one of those frustrating puzzles - metal, ropes, and rings and balls. Get the rope off the puzzle. Some are wrapped in plastic.