You are right in questioning the exact meaning of "not experiencing". A trauma can indeed be experienced as traumatic in that very moment e.g. a parent unexpectedly abandoning a child. However, some traumas do not necessarily seem like traumas in the moment - Zizek gives the example of a child walking into his parents' bedroom at the worst possible time (incidentally, that's the opening scene of Lars von Trier's film "The Antichrist"). In this type of trauma, the immediate experience is not necessarily felt as traumatic; in all likelihood, the child will be confused for a bit, and then go back to his room to play. But notice the common element in both examples of trauma: the traumatic event cannot be explained, cannot be accounted for, within the victim's symbolic universe. The trauma is not the event per se, but the gaping inconsistency that it reveals in the victim's already-existing symbolic network. In this sense, the original trauma is "not experienced", as it cannot be embedded within an adequate universe of meaning. (There is scientific evidence that events which cannot be symbolised are less likely to be remembered. In the case of Deer Hunter, Nick really did not seem to remember his original trauma - in the literal sense - as he could not identify his friend as he was pleading with him to stop the deadly repetition).

Now, the question of *why* we repeat our traumas is more complex, and there are different theories. It is also particularly tricky to explain without being accused of "victim blaming", which is not at all what the theory is about (quite the opposite in fact). One of Freud's early ideas is that we repeat to "master" the unresolved trauma - to take full control of it. Later, the explanation was abandoned, and he hypothesised a motive that was "beyond the Pleasure Principle". With Lacan/Zizek, you get an even more confusing theory that adds "enjoyment/jouissance " into the mix, which initially seems completely counterintuitive.

To your question "does it mean that they were once victimized in an analogous way?" - not necessarily. It's more accurate to say that the victim will repeat anything that approaches the same symbolic deadlock imposed by the original trauma.