Vegetables – Why would hot-packed green beans go bad when still sealed

Jars burst either because of internal pressure or thermal shock.

There are a couple things you can do to reduce the risk of bursting.

• use actual canning jars.
  Some people like to reuse regular jars that aren't as strong. This will often work fine but increases the risk of bursting.
• make sure the jar is hot.
  A cold jar into hot water or a hot jar into cold water will burst almost every time.
• bring water to a boil after adding jars
• don't overtighten lids.
  Rings should be tightened just enough to firmly hold the lid on. Too much and pressure won't be able to escape during boiling. Remember the bottle is sealed by pressure-not by the ring.

Based on your description, if you had any of these problems I would suspect the last.
However, sometimes bottles just burst because of manufacturing defects. This doesn't happen often but it is sad when it does.

This is another case for anthocyanins, the same compound that causes garlic and ginger to turn blue. Anthocyanins are pH sensitive, only appearing blue or purple when in the presence of acidity. As you cook the beans, the cells begin to rupture and lose water, which causes the acid around the anthocyanins to be diluted. As this happens, they lose their color allowing the green chlorophyll (which was always present) to show through.

The Science Behind Purple Beans