If chapter 0 is to be of any indication, the original owners were pretty much in love with each other, not to mention that the friend commented in chapter 1 how hard it was to get them together in spite of them being obviously into each other, so whatever could have caused the "accident" was something born either out of deep jealousy and/or misunderstanding or somebody else "made them" die for each other...
...because remember, when the entities took their bodies, there was no knife in sight anywhere around them.