The protagonists of both series are seemingly normal dudes who are in fact carrying hidden burdens. They both saved girls afflicted with supernatural effects and ended up going out with each other thereafter, early in the series. Both girls are strong with a cold, logical exterior but are also very loving towards the protagonists in their own ways. After handling the first incident, the troubles continue. More girls continue to be affected and the MCs play the role of the knights in shining armour coming to their rescue. In either case, at least one of these girls is a junior of the protagonist. At least one girl cannot be seen by anyone but the protagonist. At least one of these girls would fall in love with the MC after the MC is already in a relationship, causing a supernatural phenomenon. Given how similar they have been, I won’t be surprised if there’s an arc involving Araragi Koyomi’s family. In fact, the potential trouble has been alluded to when Araragi was being avoidant of his family on Mother’s day.

There have been plenty of clues that something is up - such that one can make a fair guess at what’s going on - but the twist still felt so well-executed regardless.