If I were to guess, I'd say the two starting electors that are doing well in OP's screenshot (Cologne and The Palatinate) somehow got into a war with the Emperor. Austria has some Bohemian territory, which implies they've fought at least once. Saxony is a opm, so perhaps they were allied to Austria (or whoever was emperor at time of dismantling). And Brandenburg could have been annexed while the emperor was busy fighting this other war. And Mainz controls one territory, which isn't Mainz.
So my hypothesis is as follows: Saxony or Mainz became emperor, both allied with Trier. Cologne and The Palatinate were allies, and ate up the small neighbouring nations. They then declared war on Trier, bringing them into war with Saxony and Mainz as well. At the same time, or a bit beforehand Brandenburg gets into a war with either Sweden or Lubeck, and gets completely wiped out. Same for Bohemia against Austria. Or Bohemia could be part of the Cologne war and be at war with Austria, with The Palatinate being the one to siege down Prague.
So Trier gets occupied first, then 3? maybe 4 allies siege down Mainz, Prague, and Saxony at roughly the same time. Or close enough to avoid seperate peaces. And Brandenburg is already out of the picture, not yet replaced.
This is just a theory I made up of course. OP could check province histories of the elector capitals, might give clues to what actually happened.
