Hi! I do research into protein aggregation and LLPS (liquid-liquid phase separation). Other people have already mentioned that pH isn't that different, so I'd like to elaborate on the phase separation. It's unlikely that tau phase separates itself without other components (simple coacervation), but rather complexes with more molecules (complex coacervation). Because of this, LLPS can be triggered through concentration of either component or other solutes present in the cell, or perhaps even by the presence of sequence specific proteins/RNA.

Furthermore, currently the oligomeric, soluble species are being pointed at as the main toxic species. This is not to say that the fibers don't have a role in disease progression (perhaps through spreading and fragmentation), just that they carry less toxic effects than the oligomers.

There's also the liquid-to-solid transition that's bejng discussed at the moment in literature, or my favourite, a pathway in which pre-existing LLPS compartments can influence and trigger protein aggregation.

Disclaimer: written after 12, so may contain some inaccuracies!