I suppose that many Bug type attacks were just given that type because Bugs needed more attacks and that's it.
What confuses me more are some status moves, like why is Wide Guard a Rock type move, or Coil a Poison type move.
Or why is Agility a Psychic move? It relaxes the body, so it has nothing to do with psychic powers, and in fact most of the Pokémon that learn Agility have literally nothing to do with the Psychic type. I mean, the type is irrelevant for most status moves, but still, it feels so random :)