Instant self-bounce/hit of a Ninja Rope weapon drop
From Worms Knowledge Base
This glitch occurs when a worm using a Jet Pack or Ninja Rope is descending and travelling in the same direction as a weapon dropped from the rope or Jet Pack. What happens is that the weapon spawns above the worm in play, and then hits the worm. If the weapon is an impact detonation weapon, it will explode the worm imediately. If it is not the case, the weapon will bounce and fall in a way that possibly was not planned by the player. However, some experienced players may use this phenomenon on purpose, e.g. dropping a grenade behind.
Avoiding the phenomenon
From the Jet Pack
Avoiding this self-hitting while on jet pack requires practice using the arrow keys. Back-facing weapon drops (releasing the weapon with the worm facing back) usually works very well. If you fire the weapon facing forward, sometimes using only the up arrow key is better than using two at the same time when you are about to release a weapon, but it varies depending on the situation, i.e. the wind, the gravity, the speed, etc.