There are a number of reasons the EPB can apply automatically. Most frequent is when parked on an incline of >2 degrees.
Other systems can activate the Parking Brake too, these include:
Opening Drivers door while in gear
Turning off car without being in Park (will automatically shift to Park and engage EPB)
Opening the Hood/ Bonnet
Opening the Liftgate with engine running
And some other reasons.
You can release the parking brake MANUALLY by pressing the brake, and using the switch, or the EPB will release AUTOMATICALLY under the following conditions:
1. Start the engine.
2. Fasten the driver's seat belt.
3.Close the driver's door, hood, and liftgate.
4.Press the accelerator pedal while the gear is in R (Rear), D (Drive) or manual mode.
While I know that some people will say 'they want the EPB to release automatically with the seatbelt unplugged - that's not 'normal safe driving conditions'. You can release it manually though with your foot on the brake. (Note if you open the door again after this, it will reengage).