The short answer is that we don't know exactly. And we cannot reliably predict them. Solar physicists are working on figuring out how solar flares form. It is an active area of research.
Solar flares are brief, enormous outbursts of power.
Sometimes the flare goes temporarily out of control and loses equilibrium, becoming hotter than the core of the Sun itself for a short period of time. One simplified picture that the scientists have for the cause of a solar flare is the following. Imagine the magnetic field on the Sun as loops like rubber bands that wrap around the Sun, with one end attached to the south pole and the other end attached to the north pole. Where the magnetic field snaps is where active regions on the Sun form and where the flare erupts on the solar surface. But this is just one plausible scenario, and the scientists are not sure about the details of that picture.