That's a good question and presumably you want to know to avoid a repetition.
Just out of interest, are the two failed coils on the same pair, and if so were they the front or rear pair?
I don't believe Vmax coils are particularly prone to failure hence I doubt there has been any investigations as to causes. You can search the web as to what are common causes of ignition coil failure so I won't quote that.
The coils are made of enamelled wire, the enamel coating can fail to mechanical damage, repeated heat cycling or ageing to cause a short circuit. This would explain the 0.5 ohm. The current through the transistor connected to 0.5 ohm instead of 3 ohm coil has gone from 4 A to 24 A which exceeds the maximum current for the transistor resulting in permanent damage.
The 170 ohm coil is harder to explain but my best guess an internal solder joint or contact has gone bad.