Hi Mary,
Well, we don't know exactly. There are several components to a solar storm or space weather in general. Basically the Sun generates flares and coronal mass ejections (CMEs), most likely these are a result of releasing magnetic energy by the Sun reordering its magnetic field lines to get them less tangled. We're not sure how exactly that happens or what causes it to happen but people have ideas about it. Large, fast CMEs generally go with large flares and usually a bunch of energetic particles are created as well. These particles (called solar energetic particles or SEPs) move away from the Sun and if there are enough of them they can cause problems for spacecraft and astronauts. If the CME reaches the Earth it can start a geomagnetic storm (which is basically a disruption of the magnetic field surrounding the Earth) and if there are SEPs near Earth at that time they can get in and cause problems with airplanes, communications, and power lines.
Because of the effects of geomagnetic storms, many scientists are working hard to understand how the Sun generates flares, CMEs, and SEPs so that they can be predicted (like regular weather) and precautions can be taken.
Best,
Christina