Candlekeep forums are the best place to go to look for lore on the Forgotten Realms. For the most part, however, each returning god has his or her own reasons. The Sundering novels covered the resurrection of one god, the sun god split in another. The return of Eilistraee and the other drow dieties was actually planned out in the original novels where their death occured - the writer left room for them to return if necessary. Mystra was revived by the actions of Elminster.
In general, however, the basic idea is that Ao decided that it was to happen during the Time of Troubles, and planned for it when he broke the Tablets. The overgod made sure everyone had a chance to come back, and most of them did through various efforts.
Here's the thing with the second Sundering. Unlike the Time of Troubles, there was no single cataclysmic, world changing event. Rather, the Sundering was just a very large number of small events happening simultaneously. Succubi left Asmodeus service to either go back to the Abyss or independant which restarted the Blood War, lost lands returned, the Weave was restored when Elminster revived Mystra (ending the Spellplague), gods decided to fill in the Sea of Fallen Stars with lots of rain, the Many-Arrow orcs decided to rampage, etc. Just a million smaller things happening around the same time.
So, the gods, even dead ones, had an opertunity to arrange things for their return through their Chosen. And the vast majority of them did.
In general, however, the basic idea is that Ao decided that it was to happen during the Time of Troubles, and planned for it when he broke the Tablets. The overgod made sure everyone had a chance to come back, and most of them did through various efforts.
Here's the thing with the second Sundering. Unlike the Time of Troubles, there was no single cataclysmic, world changing event. Rather, the Sundering was just a very large number of small events happening simultaneously. Succubi left Asmodeus service to either go back to the Abyss or independant which restarted the Blood War, lost lands returned, the Weave was restored when Elminster revived Mystra (ending the Spellplague), gods decided to fill in the Sea of Fallen Stars with lots of rain, the Many-Arrow orcs decided to rampage, etc. Just a million smaller things happening around the same time.
So, the gods, even dead ones, had an opertunity to arrange things for their return through their Chosen. And the vast majority of them did.