Psychological explanations (e.g. memories are fallible), metaphysical explanations (e.g. the Universe as we knew ended on December 21 2012 due to the Higgs Boson and it has been replaced by a simulation by CERN), and possibly others (even unorthodox/bizarre).
1: Psychological explanation: memories are fallible. There may be several reasons why a large number of people has false memories. In this explanation, the memories are false, they refer to events that didn't actually happen.
2: Idealistic/semi-solipsistic explanation: "reality" is not as stable as one may think, and consciousness may alter reality. The memories in this explanation are not strictly "false", but an alternate reality that used to be real but got "removed" from "canon reality" (though not from many people's memories) and became non-canon (though it used to be canon when it first happened). Here it's a matter not of false memories, but of transition from canon to non-canon.
3: Cosmic apocalypse explanation: the Universe as we knew it ended on a certain date (most popularly in culture, on December 21 2012) due to a certain event (e.g. the Higgs Boson), and after that date:
3.1: The Universe has been replaced with some sort of non-physical simulation (possibly by CERN).
3.2: Nothing exists anymore, everything has been annihilated. However, since nothingness is a logical and philosophical aberration and cannot exist by definition, random qualia appear to be generated (though not even those exist, it's just an illusion) in order to fill "the void", even though the truth is simply that nothing exists anymore, as all matter and energy and everything else has been annihilated, leading to the end of the Universe.
Other explanations may be coming soon, if I have them in mind.