Pelican Parts Forums - Galaxy 7- It's the phone, not the battery

Not enough space designed into it:
Engineers have finally worked out why the Samsung Galaxy Note 7 kept exploding - Mirror Online
"Any battery engineer will tell you that it’s necessary to leave some percentage of ceiling above the battery, 10% is a rough rule-of-thumb, and over time the battery will expand into that space.

"Our two-month old unit had no ceiling: the battery and adhesive was 5.2 mm thick, resting in a 5.2 mm deep pocket. There should have been a 0.5 mm ceiling."

The lack of "ceiling" meant that, as the battery expanded, it ended up getting pushback from other components, increasing the pressure inside the case itself.

That pressure is believed to have made the separator layers between the positive and negative portions of the battery more likely to rupture, causing a reaction within the battery pack.