Not to put too fine a point on it but at the core it is a plumbing problem…in part. Following link is a great overview of the glymphatic system, which only recently came into focus as its structure was largely hiding in plain sight. Not hard to imagine that amyloid beta deposition might be enhanced if you can back up the plumbing system. Note in particular figure 1, which shows that astrocyte endfeet form a virtually complete ensheathment of the arterial vasculature; also note the perivascular space created is allowing bulk movement of CNS fluid. Further note figure 2 which shows factors which can enhance or impede this fluid flow…vasoconstriction enhances…and astrocyte endfeet swelling impedes this flow. This fluid super-highway facilitates chemical signaling among other functions.

https://www.jneurosci.org/content/41/37/7698

Microglia are in close proximity to both neurons and astrocytes; when activated they release TNF-alpha and IL-1beta, which diffuse to neighboring astrocytes. These microglial effectors are capable of massive overproduction of CCL5 by astrocytes. You can readily appreciate from figure 2 how this pro-inflammatory state would influence perivascular fluid flow dynamics.