This is really long, and I only had time to scan it and read the intro and conclusions and pause to look at some pretty pictures.

I'm putting it here for reference to dig into it later.
https://www.pnas.org/doi/10.1073/pnas.1908065116

Seems that it all comes down to shear strain moving the suspended particles into close contact with each other, where friction between those particles can dominate.
Basically, when the fluid tries to move out of the way, it causes the suspended particles to touch each other and lock up. That locking up stops the shear strain, and the particles relax back into the fluid, losing their contact forces with each other.