This relates to the now question. The reason that time travel creates paradoxes if that it violates an implicit assumption, a definition of time.
Sequence is the core to our definition of time.
Time is the structure that causes us to say that one thing happened before another. In other words, time is the stuff you can't go back in. So it shouldn't surprise anyone when time travel creates paradoxes. OK, let's say you go back in it. Hmmmm . . .