If it's a toyota, bad manufacturing. Something might get stuck underneath, like a mat (happened to me once, of course in my toyota, which is another issue they had). Basically, having a Toyota tends to cause this. Go get it checked out. It's not pleasant to have a car moving on its own.
The accelerator cable may be frayed inside the sheathing. This happened to me and I had to drive with the wheels spinning and my foot on the brake until I got home. Fortunately it wasnt far.
Just a random guess; slamming on them too much OR you push it in to far. Depends on where it is.