There is no chain reaction taking place. The reason they pop can be found in the design of the popcorn kernel. The kernel is protected by a hard outer shell or hull. Within, water is enclosed. Under sufficiently high heat, the water turns into steam, 4/6  BIS central bankers' speeches  raising the pressure inside the kernel.
At a temperature of approximately 180 degrees Celsius, the pressure becomes strong enough to burst the shell.18 We could compare the pressure inside the kernel to financial problems building up inside banks, while banks’ capital may be thought to resemble the protective hull. Undercapitalised banks were a salient feature of the pre-crisis financial system. Ultimately, they were unable to sufficiently withstand financial stress.