The question posed is about welded parts that fail - pop open while sitting on the workbench for a few minutes. The parts were probably distorted and stressed during welding. As they cool the shape tries to return to its original state. This puts stress on the weld joints. Poor joints could fail. There are two general areas to discuss as the cause of the weak joints. One is the work piece material composition. The other is the general weld procedure and resultant nugget.
Brittle Fracture in Parent Metal
If the work piece is a high strength steel. It is very possible that the weld nugget and area around it has been quenched during the welding process and is now a brittle martensitic structure. It would definitely be subject to a brittle failure. To eliminate this condition, a tempering or slow cool sequence must be incorporated into the weld schedule to eliminate the brittle martensitic structure.
The other possible problem is the weld nugget itself. When welding any material with or without a coating, one can get a stick weld. This is where enough heat was generated to bond the surface together without any heat/nugget penetration into the body of the material. If welding galvanized material, the Zn coating can actually bond together and hold the parts together. It is essentially a braze joint. The joint is weak and will fail when the first amount of stress is placed on the part. Proper weld nugget testing can find this condition and indicate the need for schedule adjustment to get a proper nugget.