Sounds to me like a classic "unbalanced pack" problem.
The old batteries had a much lower capacity than the others, so they were depleting faster than the newer ones. When they "blew", I suspect that was the result of a cell reversal - the voltage in an individual weak cell was drawn down to zero, at which point it reversed and started being charged by the neighbouring cells. ... At 100+ amps, or whatever the current was while you were driving!! That much current will generate a lot of heat and gas, and a BANG! isn't out of the question.
I'm a firm believer that all EV's need pack monitors: either passive (by computer, with appropriate warning methods) or active (some kind of array of gauges the driver can keep tabs on).
The battery pack is usually the most expensive component in the car, so it just makes sense to monitor it in a more sophisticated way than a single "state of charge" voltage gauge.
EG see:
Video: LED battery pack monitor / bargraph
You can also get commercial versions of that: Pak-trakr is one.