When Ao the Overgod destroyed the Tablets of Fate at the conclusion of the Time of Troubles, he instigated the Era of Upheaval. The Tablets defined the laws of Realmspace and kept it relatively stable. Without them, chaos ensued and the worlds of Abeir and Toril, separated many thousands of years ago, slowly started to overlap. The Spellplague drastically sped up this process.
The Second Sundering started with Ao's decision to recreate the Tablets of Fate, and to separate the worlds of Abeir and Toril once more. The deities were unsure of what this would do to them and their power, and made efforts using their most powerful servants to prepare for that event.
Tldr; Big Reset