Report: Samsung's aggressive battery design led to the Galaxy Note 7 explosions
As the report points out, compressing the battery would force the positive and negative layers to squeeze together. Samsung itself said that it might have been too aggressive with the design. In addition, there is a normal amount of swelling that takes place. Some of it comes from the pressure on the battery created through pushing on the back cover (for example, by putting the phone in one's back pocket and then sitting on a chair).
In order to make the phone thin, the batteries were designed to be carry as much capacity as possible, while keeping to a svelte footprint. The report stated that if the phablet had not been recalled, the body of Galaxy Note 7 units throughout the world would probably have separated.
Samsung was taking a risk by shipping a battery that left very little wiggle room for problems. And yes, this should have been spotted in tests. However, Samsung broke protocol by handling the testing itself.
Samsung did shoot itself in the foot. By one calculation, the company will tally more than $5 billion in lost revenue and other costs associated with the recall of the Galaxy Note 7.