can anyone explain to me why back to the future part 2 is a paradox?

Answer 1

If you could go back in time (which you can't), it would create paradoxes because you could go back in time and kill your grandfather so you'd never be born, so you couldn't go back in time to kill him, so you would be born, ....

The back to the future series plays pretty fast and loose with the idea of paradoxes. The Michael J. Fox character just started fading slightly when he was creating paradoxes.

Answer 2

There is a WAG, some physicists propose, that there is no paradox. Their WAG is that there is a law of physics in our universe that will prevent a time traveler from going to any time and space where the future might be altered from its original path. That law of physics, which I will call the mutual exclusivity law, has yet to be discovered as no one has yet traveled back in time.

Thus, for example, suppose you could take a worm hole back to the time you were born. The mutual exclusivity law would not allow you to go anywhere close to your mom or the time of your birthing. Maybe, you could get to somewhere on the other side of the Earth from your home town and maybe you could get to your hometown a month after you were born, but you would not end up where you could change your birthing and the future.

For more fascinating WAGs on time travel, check out "The Elegant Universe" by Brian Greene of Columbia U.