Voltage reducer - loud pop and trips lithium cells

[demarcor]

So… just put 2 BigBattery eagles in my stock ‘06 precedent. Drove 8 miles in hilly terrain and lost only 1 bar on the meter! Very impressed so far.
Problem…
My voltage converter unit, when wired into the busbar that BigBattery provides, won’t allow me to power on the cells. I disconnected the reducer, and then both cells turned on, no problem. With both cells on, I attempted to connect the little ring terminals I crimped onto the reducer onto the busbar… and POP!!! Huge spark, trips both breakers. Everything appears hooked up correctly…. What the heck is going on??? It’s a 48 / 36v to 12v reducer off Amazon. Silvery looking box with fins. Headlights and radio wired to it.

Whaaaaat???