Most of the gadgets we report on use lithium-ion batteries. And like all batteries, they store chemical energy. In most cases they're extremely safe, but if something happens to make them release all that energy at once, what you get is a tiny trouser grenade. At least one owner of the OnePlus One is reporting that his phone went kaboom while he was walking in the subway, destroying the hardware and giving him a minor skin burn.
The poster "MiYzu" claims that the battery exploded in the back pocket of his jeans with no external stress of any kind. Sudden impacts or extremely high temperatures can cause just about any Li-ion battery to malfunction and release chemical energy, but if it happens without warning or reason, it's generally assumed that there's a hardware defect involved. Various large-scale recalls of lithium ion batteries have been instituted in the past for that reason; the biggest was probably a series of Sony-branded defective OEM laptop battery recalls in 2006.
Samsung was in a bit of hot water last year after a Galaxy S3 battery exploded and seriously injured a young woman, but an investigation revealed that she was using a third-party battery with a counterfeit Samsung logo. That's almost certainly not the case here, because the OnePlus One's battery isn't removable (though it is physically accessible thanks to the swappable back cover).
At the time of writing at least one OnePlus representative has reached out to the original poster via the forum, and the OP replied that he has submitted a support ticket.