I'll look further later on this.
All i wan to say now is, i've been in this configuration since i installed pi-hole on home network last August. Also i've been on Zcalar on and off since then. All was well, until i upgrade to the latest version for pihole !!!
Pi-hole doesn't generate DNS queries on it's own. You have a client that has decided that it really needs to know Zscalar's PTR records. This isn't a Pi-hole issue, it's a rogue client or a bad ZScalar application.
This indicates that you have a very chatty client. That is where I would focus the investigation - why did that client suddenly making the same request over and over.
Update:
After flushing ( pihole -f ) both pihole (one on each Raspi), chatty client this time around was webex.com. The machine (work) that was running webex was disconnected from the network. Total Queries kept on climbing for quite sometime, until router DNS was changed ! This tells me the loop (chatter) was between the router and pihole. Where do i go from here ?