It seems plausible to me that when the phone came out of airplane mode, it increased radio transmission and/or CPU use. Due to some fault in one of those systems, the thermal throttling didn't kick in, and caused enough heat to melt open the battery. Perhaps the phone was jailbroken and overclocked?
I find it both amusing and telling that that is the only possibility you consider. Perhaps there was a manufacturing flaw in the battery, or it was damaged by dropping, yet 'blame the user' seems to be your knee-jerk reaction.
I started this by saying "Perhaps" the phone was jailbroken. I don't assume that is the case, I just provide it as a plausible option. Also notice that the Apple logo on the phone back has been modified.
