Lots of memories are constructed and modified post hoc, sometimes confabulating about events that you cannot have witnessed, or that cannot have formed memories from. (Two famous examples: Memory of seeing both twin towers collapse one after the next as it happened (when it fact the latter was shown only after a large gap), memory of being born / being in the womb.)

I'm not positing that you can have causeless memories, but there is a large swath of evidence indicating that the causal experience does not have to match your memory of it.

As a thought experiment, imagine implanted memories. They do have a cause, but certainly their content need not mirror the causal event.