I felt it was proper to choose what I thought was the most likely date (if I could) in order to best teach/preach the book. I settled on an early date, indeed I am of the opinion that Joel is the earliest of the writing prophets.
The specific crisis that stands in the book's immediate past--not the "motive" for writing, but an aspect of Joel's subsequent ministry--is the throne-jeapordy that takes place when Ahaziah king of Judah dies, and queen-mother Athaliah slaughtered the sons and heirs of David in order to perpetuate her own power, 2Ki.11.
The idea of such a powerful threat to the Promise was unprecedented in history up to that moment. There were other threats, other crises that had come (and would arise, rivaling this one for desperation). But David's house seemed to hang by a thread, in fact to all but a tiny handful of witnesses for a time it did appear to all observation the kingly line was cut off.
Clearly, faith would not doubt the LORD must maintain his plan of salvation, and still would fulfill his promise to David concerning his son (Son) and heir. Clear vision of the reality of danger, but also powerful hope calls Joel to serve the church in that dark hour. He identifies serious problems that must be faced and admitted by the people of God and its leaders; but he also declares the unwavering purpose of the LORD to do good for his covenant race.