When was the end of the Rishonim time period and the start of the achronim time period? Who, what, why, and how did this come about? What caused this abrupt change in deemed mental and halachot significance/intelligence?
Really the distinction is one between the medieval period and the modern period. The Shulchan Aruch was written at the very end of the medieval period and was one of the first Jewish books to be printed during its author's lifetime. The printing aspect is hugely important; before that any Jewish work (any work in general) was competing for scribal time & effort.
The shift happened from the Shulchan aruch.
Before the Shulchan Aruch, halacha was decided by each posek looking towards the Talmud and Geonim, or being influenced by other Rabeim, and coming to their own decision about an issue, based on what made sense to them and their community. (Using many many factors)
The Shulchan Aruch however, decided to take a 2 out of 3 approach (best of Rif, Rosh and Rambam) This was a new method of psak halacha.
I imagine there is also a shift due to the Arizal, but I have no source for that.
rishonim were the leading rabbis before the shulchan aruch and achronim after shulchan aruch (5323-5324)
The time of the shulchan aruch is about right; there is also a shift in rabbinical names about the same time.
many(but not all) rishonim went by acronyms...rashi, rashbam, rambam, ramban, or by their own names .... ibn ezra,sforno, abarbanel
acharonim often wrote their comments under a pen name....chafetz chaim, gur aryeh, etc.