I will chime in with my two cents for whatever it may help. In 12 + yrs installing caraudio and alarm systems I have seen quite a few "mysteries" with speaker failures. Majority of the time it was one of two things : distortion via being driven by a underpowered system, which basically boils down to trying to achieve higher volume levels than what the source unit is capable of CLEANLY, most head units are rated at a max power rating, clean power is way less but it is also much cleaner, where as why everyone adds an amplifier of some sort to "amplify" the clean source signal. 2nd majority cause of failure is installation, whenever a speaker is mounted through or to a structure sometimes they are put in a bind by either improper hole sizing or abnormally unlevel susrface which tweaks the frame and causes the voice coil to drag or rub until it fails. As far as them replacing with a 2ohm setup over 4 ohm setup, if head unit is capable of 2 ohm load it will increase power output of head unit as well, but rarely are any factory headunits setup for 2 ohm loads. If they replace and same one fails again i would definitely look at installation area, even switch the one over that hasnt blown yet and see what happens. Good Luck!
2013 Cross Country, RJC exhaust, Lloyd's Filter, Timing wheel, PCV, Adjustable Rear lowering link, Goody's front lowers. NW New Mexico