Why does a RYOBI 18V ONE+ HP Brushless 30-34° Framing Nailer turn off by itself?

The RYOBI 18V ONE+ HP Brushless 30-34° Framing Nailer turns off by itself as a safety measure. 

You need to activate the tool each time you replace the battery, or if it’s been idle for 20 minutes.

To activate, see: How to turn on & activate a RYOBI 18V ONE+ HP Brushless 30-34° Framing Nailer.