What causes a meteor shower?

There are tiny dust-sized particles in Earth's path around the Sun. These particles are usually leftovers from asteroids or comets.
When Earth passes through these particles, they burn up in our atmosphere producing bright lines. This is called a meteor shower.
. . .a group of meteors that have an orbit that intersects the orbit of Earth, resulting in a large number of meteors entering the atmosphere in a relatively short span of time. Said another way. . .

Meteor showers typically come about due to temporarily high concentrations of sun-orbiting debris that crosses the orbital path of Earth and, hence, collide with her atmosphere.

While there are no "comet groups", comets being invariably solitary, debris cast off from a comet during its close approach to the sun can participate as "meteoric" space junk that encounters Earth during a meteor shower. Meteor showers tend to arise from old comets that have broken up and spread out along the comet's orbit. A meteor shower happens when the Earth's orbit intersects one of these old comets' orbits. That is why the same shower happens around the same date each year, and the meteors in that shower all appear from the same 'radiant point'.