You can't be electrocuted by the low voltage in a charger. But you can be electrocuted by a fault path from the AC line.
I can think of two ways for this to occur. The first is a fault through the insulation in the charger. Frankly, I think that is unlikely, because the chargers are built with two layers of insulation. Another way--the oops way--is if the charger was on an extension cord, and the extension cord fell into the bath with the charger. Then it's the same situation as if a live AC wire was placed directly into the tub. I wouldn't be surprised if that's what happened.
In modern houses, there are ground fault interrupter outlets near things that can provide an expedited path to ground... like near the kitchen or bathroom sink. There's no way to know if that was the case here.
My bet: The extension cord fell in with the charger.
A fault in the charger that allowed one side of the AC mains voltage to appear on the USB ground is the mostly likely failure mode. There are big differences in design and quality between a true Apple charger and some cheap chinese phone charger too.
Since it happened around bath water, if the cord was wet it may have provided an exterior path from the outlet.