A charismatic fanatic.
Give them a leader with very strong ideals, and instead of making him evil and destructive give him personality traits that would make it easy to follow. Strong, smart, fair, self-aware, humourus, and able to take and keep control over the majority of people. And his mind is set strong on that his way is the right way and the only way.
Playing into his hand is that people feel lost and insecure about their new home and they are desperate for someone to tell them what to do. Maybe many people suffer homesickness. Or some other reason that makes them feel bad. Let him lead "a new way" that discards old beliefs and knowledge, with the promise to feel good when they do it. 425
This way does not necessarily need to go backwards, like the destruction of technology and living primitive, but instead a leap forwards, making current technology and knowledge irrelevant or seem naive.
What that could be in detail is up to you. In any way it leads to him deciding that old contacts are contra productive (like keeping the relationship with your ex alive; why are you hurting yourself? You know this is not leading to anything) and most people will join him willingly and throw out the past. But of course there are always people going against the mainstream (out of multiple motives) and you have to decide what to do about them. Do they have to die? Or can this small fraction form a new settlement and are allowed to keep the old ways alive? Are these two groups allowed to keep contact with each other?
If you want to discard of any contact to civilasation you can get rid of the drop-out in multiple ways. They could have a real accident, or an "accident" either arranged by the leader or by his followers of which he does not approve (but he only hears about it when it is to late). Or they can go back home.
And then you let three or more generations pass to make sure all "histories of before" are only fairytales.
If you want it to be more extreme, old histories will be forbidden and destroyed or alternated. But be aware that the reader will expect "some little bits and pieces" of previous knowledge to remain and to be rediscovered. 159