Are there proposed reasons for increased blood flow to brain regions other than neural activity? Are neurons flushing waste products or something when less active?

Many reasons, and yes, basically, that is one of them.

Ekstrom, A. (2010). How and when the fMRI BOLD signal relates to underlying neural activity: The danger in dissociation. Brain Research Reviews, 62(2), 233–244. https://doi.org/10.1016/j.brainresrev.2009.12.004, https://scholar.google.ca/scholar?cluster=642045057386053841...

The glymphatic system, sure.