Thor dropping the hammer happened at the end of the series. So it has something to do with the events happened during the series. Most likely something Thor did.
- Nick Fury told Thor that Kid Loki had been murdered by Old Loki, and
that when he took over that body Thor didn't even notice. Not
realising that the nice kid who wanted to beat the fate of his name
had been murdered and replaced has made him unworthy in his own eyes.
Kid Loki is just some kid who is the 'new' Loki, while Old Loki still existed as a disembodied spirit. When Kid Loki was tricked into killing himself by Old Loki (which he knew would happen, he just wanted to save Thor from Ragnarok), Old Loki took over that body.
Shortly after that Kid Loki grew up a bit in Young Avengers, and is currently still that age in Agent of Asgard. While he still is the same Loki that will one day commit Seige, he's trying to change his destiny, based mostly on his guilt over killing Kid Loki ("I am the crime that will not be forgiven!").
Above explanation has a connection with Journey Into Mystery #625-632
Unfortunately, Thor didn't take Loki's return to trickery very well and, enraged, nearly choked the kid. A tearful Loki explained that he'd done it to save Thor (who had a strange, unhealing injury; the Norns had claimed that if Loki did as they bade, Thor would be saved) and everyone else, despite how they all hated him. He then accused Thor of being just like the others, saying that it didn't matter what (Loki) did and they would hate him regardless.
Later, Thor tracked Loki down and aplogised for losing his temper, assuring him he loved his brother — but also admitting that he didn't trust the kid.
Whatever Kid Loki did, Thor never fully trusted him. In the end, Kid Loki died but Thor didn't realise that. That was the reason he felt unworthy in my opinion.
- Nick Fury told Thor that he caused Jane Foster's cancer.
Now we know that Jane is the new Thor. When Thor realised that he became unworth and Jane became Thor.