Who came with this stupid bridge rectifier idea? Any thermal design being done? It's not rocket science! Doesn't this guy know that diode current rating is meaningless? I * Vf * (Rth_j-c + Rth_c-hs + Rth_hs-amb) matters. Claim that putting 50A through 100A rated diode bridge is "conservative" shows this guy has no freaking idea about the most basic electronic design work. That stupid bridge dissipates some 100W at 50A. Where is it located? How about heatsinking? Being like three-four soldering irons set at full power, it's well capable of externally inducing thermal runaway on the pack if located close. Worst of all, he's getting correct advice from the battery pack manufacturer, saying putting these battery products in parallel, series, or using such bridge rectifier circuit, is dangerous and shouldn't be done. But he doesn't listen because he has mixed up cells and batteries himself. Dunning-Kruger is really showing in this video, and the fan echo chamber really doesn't help.
In addition to crappy circuit design and construction techniques, I'm suspecting the authenticity of the cells. Buying a random, no-name, poorly constructed Chinese pack with "Samsung 35E" cells, why would you expect to receive anything but counterfeits?
Proper cells have quite some passive safety. You can set them fire if you totally fail the design (especially by heating them up beyond some 150 degC with external heat source, such as failed electronics), but it will happen way easier with crappy cells lacking passive cell-level safety.
Who knows what happened. My impression on many battery pack fire videos is that they are horrible hacks where I can instantly see so many things being wrong it's impossible to say what is the root cause. On the 'net speculation, it is always because of having a BMS, or because of not having a BMS (in either cult, what the BMS actually is and how it's designed seems irrelevant.)
Issues with Rossmann's joke circuitry are undeniable; but it's highly likely such pack has manufacturing problems as well. As usual, accidents tend to happen when multiple things go wrong at the same time.
It's scary how crappy li-ion cells and packs are sold to completely clueless hobbyists; they are finicky technology which requires some real understanding at every level of design. But if you think about it, someone with a bit more understanding would do their due diligence while buying such products, including inspecting what they get.