Its making sense from what you all have put together. The main comes
in and splits. One to cold side of manabloc, the other to the Bosch -
then it goes into the hot side of the manabloc. If any cold service is
used, even if it's a hose bib, a drop in pressure is felt all of the
way into the Bosch. That reverse pressure is enough to kick it on.

Does that sound right? So maybe I need a backflow preventer?