what causes CMEs?

Without understanding the details, we can imagine that, as the surface of the Sun moves about, the magnetic field gets tangled and twisted.The magnetic field can stretch like an elastic band, but only so far. Just like an elastic band, if it's stretched too far, it snaps. This transfers magnetic energy into heat and kinetic energy...and a CME is born. Possibly!

These four pictures were taken by LASCO.

They show what a CME looks like when it is being born and as it moves out through the corona.

The CME pushes outwards as an arch of plasma leaving behind an area with less plasma than before (the black areas in the picture).

The edge of the disk that blocks out the Sun can be seen in the bottom right-hand corner.

Here are two movies from LASCO, both showing the same CME as it shoots away from the Sun. Looks a bit like a light bulb, doesn't it?

CMEs are not always alone.
Sometimes they can happen at the same time as a solar flare.

What’s the difference between a CME and a flare?