When on the ground it is not perfectly level. It tries to make it level, but there is not enough power to lift the low arm.
The PID I term says try harder and harder until it is level.
It doesn't do this in the air because a small amount of change does change the leveling.
One last important thing, if you let it wind up a long time, then when it is in the air it will take a long time to unwind and so it will flip over when you take off. The rule you should use is "take off as soon as (like within a second or two) your motors start to turn". Generally speaking that also means make it jump up off the ground instead of slowly adding power. "Jump up to knee high" is a good take off strategy.
Leave your expensive camera and gimbal and FPV (including antennas) off of the copter if you are unsure of your abilities. Don't let it get higher than your waist. Cut the throttle and let if fall before it gets into a bad situation. A cut throttle fall from waist high onto grass usually won't hurt anything if you removed the expensive, fragile components.