It all happened when an employee was trying to fix a broken screen on the phone, and BOOM the phone exploded on the desk and caught on fire.
In the video, you can see the employee jumping and running to call someone for help. The flames did burn out while she called for help.
The store owner says that exploding iPhone batteries are rare, but they do happen. Most of the time it's when they get extremely hot, so be careful not to overcharge your phone.
Tip of the day: don't ignore the "iPhone too hot" warning that pops up on your phone, like you ignore the updates.