Most of the gadgets 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, that might be troublesome.
One owner of the OnePlus One has reported in OnePlus forum 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 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.
OnePlus One has non-removable battery so there are no chances of using any 3rd party battery which may cause this kind of trouble. Moreover OnePlus One uses a lithium polymer battery, not the more common lithium ion design. LiPo technology uses a solid electrolyte, which should be more difficult to damage via external pressure.