See also also bike pic for other bike-related technical discussions and more failed parts.
The chainstays of this bicycle split, presumably when water got inside and froze.
Photo courtesey of The Yellow Jersey. The bicycle owner said the bicycle was parked during the winter under a roof edge where water dripped on it. Presumably, water got inside the chainstays and froze.
Water expands when frozen at atmospheric pressures, and is very inelastic, so it cannot simply be squeezed like rubber. If forming ice has no place to "escape", and if the container is rigid, it often grows enough to burst the container.
Bicycles are largely all-weather machines — some people find it inconceivable to ride a narrow-tire "road" bike in winter, but winter riding is common around the world and long predates the "mountain" bicycle.
Usual concerns in freezing weather are traction, braking, and whether the drivetrain or wheels will get so clogged with snow and ice as to make the bicycle unridable.
It is unclear what led to this frame freeze failure. Since both stays have burst, water presumably filled the bottom part of the frame, then froze. How did the water get in? Maybe through the vent holes that are commonly drilled near the dropouts. The vent holes let air pressure equalize during welding, and also as air pressure changes during use. In normal use, some water enters via the vent holes, but tends to evaporate, especially with heating/cooling and air pressure changes that force out moist air and bring in dry air.
Alternatively, the water may have gotten in around the seatpost. Although the post is clamped, it is not water-tight. A common problem is water thrown up from the rear wheel gets in through the "keyhole" slot and collects in the bottom bracket, causing bearing damage. Some frames have a slightly extended seat tube and put the keyhole slot at the front. That reduces water intrusion from the rear wheel, but exposes it to more water from riding in to the rain. Finally, some riders put a rubber "boot" around the joint to reduce water intrusion.
Burst frames are thankfully rare, but the failure is interesting and instructive.