I think the most likely cause of fires for batteries is the charger or BMS not being smart enough to detect fault conditions.
My company's batteries/chargers monitor temperature and current and will cut power if either one exceeds limits. They won't start charging if pack voltage is too low. (indicating a bad cell, which would lead to overcharging I mentioned above)
In eBike batteries, I don't know what is done in this regard. With a two pin connection going into the pack, there's no room for that kind of sensing, and we would hope there is some well-designed smart circuitry built into the BMS in the pack. The less expensive the battery pack and with the fewer certifications, the less I'm sure of it.
Another one that causes fires would be using the wrong charger. Let's say you have two eBikes; one is 36 V and the other is 52 V. The 52 V battery would be charging at around 60 V. Mix that up and plug it into a cheap 36 V eBike battery without good BMS built in and it's a recipe for a fire. (lithium batteries will light off if charged at too high of a voltage. It's 4.2 V per cell or parallel cluster, IIRC)