My dell optiplex 3080 computer automatically started without click power button

Maybe you have removed the power source from a live outlet?  Anytime users introduce power to a Dell system in its shutdown state, the system will power on by itself, initializes a system BIOS check and then it would stay on or shutdown again, depending on power management configurations in BIOS settings.

If this was the case, keep the system plugs into a live outlet will stop this type of behavior.

If this system was setup by a business prior to you receiving it, there's also the possibility that it was setup to power on at a certain time (a setting that can sometimes be set in the BIOS by the IT department). 

As Chino said, it's also common for Dell systems to power on for a few moments when first connected to power. I'm assuming it's doing a quick check of the hardware, and then it powers back off. 

If the system is staying on after plugging it in, but it boots to Windows like it would if you hit the power button, you're probably ok. Just test it a few times by powering it off, and then pressing the power button. If it works as expected, you're good.

Go into F2 BIOS Power Management and turn off wake when power is introduced.

Adding to what Dnev said in the 1st pgh., make sure there's no wake times set in BIOS Power Management.

As Chino suggested, keep the system plugged in if not already.