simple trigonometry.
Jack the front of the car in the air, and due to the fact that the lenght of the car is set, it WILL want to move forward on the rear wheels, or the jack will want to slide back...thats if the jack is in line with the wheels and you are jacking high enough. Seen it happen hundreds of times when jacking ALL my cars...
Chock your wheels, jack one side of the car with the jack perpendicular to the wheels and couple of things will happen, you will try and pull the rear wheels towards the jack, you'll see them start rolling, the jacking point will slide if you're just using a flat part of the chassis and a jacking pad, or the entire jack will start to be pulled towards the part of the car that is still on the ground.
I'm not pulling this out of my ***, or talking hypothetical, i have seen this happen, every single time i was jacking to a higher level than just enough to lift the wheels of the ground.
This has happened on my bimmers during engine and tranny swaps, nissans, civics, acuras, Land rovers etc etc.