Trouble is, how the heck do you investigate something like that? The only possible clue is that there was a shooting star both of them saw the night before their first bodyswap, but that's not much to go on - no one else who saw it seems to have been body swapping (as far as we know), and I can't see that either of them were doing anything when they saw the star that might have influenced it (Suno was wishing to never see Lemon again, Lemon was doing nothing).
It would be different if there was some curse/artifact/wrathful kami causing it, because that's something they could actually research, but that doesn't seem to be the case.