According to the reports, it seems that nothing out of the ordinary was happening. The phone was on the bed charging and was not covered by the sheets or a pillow, so overheating did not seem like a possibility. The person whose phone caught on fire said that they heard a sizzling sound followed by the smell of “nasty fumes”, before it caught on fire.
The only thing out of the ordinary was the fact that the person was using a Samsung Galaxy S3 charger to charge the device, instead of the one bundled in their purchase. However according to the Repair Center in Pembroke Pines Florida, they claim that it is unlikely that the Galaxy S3’s charger is to blame. Instead it seems that they are chalking this up to a possible faulty battery or damage to the phone itself that might have been caused by the user.