What causes seals to fail when canning? My husband made and canned pickles 1 week ago. Tonight he noticed the seals were popped on all but one jar. One other time the same thing happened when he canned green beans. What would be the cause of this?
Aside from the notes in SAJ14SAJ's answer, if he didn't run the canning process long enough, there may have been residual bacteria (or spores) that remained active.

The failed cannings I've seen due to insufficient heat took exactly one week to pop their lids.