It had been bought the day before from Aldi.
This morning a watermelon bought from Acme or Aldi did the same thing.
I read that the grocer was at fault for not rotating their produce properly and were selling melons that were too old and/or damaged.
Both incidents made a huge mess to clean so I am not going to buy anymore melons for quite awhile.
I decided to buy just pre-cut melon that looks and smells good and avoid anymore messes.
My husband agrees in spite of his frugal nature after injuring his arm when lifting a large melon a few weeks ago