Android is upgrading- trigger?

So im using the nexus 5 for like a year and my power button became problematic, pressing itself sometimes, ordered a new button.
And im going to replace it, untill then, i downloaded third party apps to use the nexus without having to press the power button.

My question for you guys or experts, is what trigger makes my phone sometimes (after pressing the button which makes reboot) to go into the "android is upgrading optimising apps", and if it is a manual trigger with the power button, is it possible to turn it off?

Thanks in advance