Kscreenlocker doesn't seem to be aware of input from USB gamepads, so it will lock if only those devices are being used.
 
I noticed this happening after I upgraded from KDE 4 and was playing a full screen game through Steam while using one of my USB gamepads.

The default settings was set to 5 minutes, so the screen lock would interrupt my game after every 5 minutes. The obvious workaround was to turn the screen lock feature off, but I don't think that will be the best solution in the future.

Reproducible: Always

Steps to Reproduce:
1. Upgrade to KDE plasma
2. Ensure that the screen lock feature is on
3. Launch a full screen game, but only play with a USB gamepad
4. Avoid hitting the keyboard or using a mouse

Actual Results:  
Kscreenlock will interrupt the game when it wasn't necessary since USB input was being passed into the OS via a gamepad.

Expected Results:  
Kscreenlock should account for USB accessory input and not automatically lock.