What causes canned goods to swell and/or even explode?

Here are some possible causes: damage to the can during processing, overfilling or underfilling of the can, or sealing it after it had cooled down. Denting caused by rough handling after the can was sealed might also be a factor. Other culprits could be high summer temperatures or high altitudes. The main causes, however, are microbial spoilage and hydrogen produced by the interaction of acid in the food with the metal of the can. Pressure is exerted on the can, causing bulging at both ends; if the sealed can is left on the shelf indefinitely, eventually it may explode. Do not wait for that to happen. At any sign of swelling or major damage to the can, return it unopened to the store or discard it.
 
Source(s):
Canned Food Alliance e-mail interview
Neal Jones Company (cannery) phone interview