That's normal operation. If you prefer it to come on without using the power button check your BIOS for a setting for A/C power recovery or similar and set it to turn on after power loss
When my 7010 SFF is plugged into the power bar and does not press the power switch, it will run for 1 or 2 seconds and then go out instantly. You need to press the power switch again to start the computer. What's the problem with it? Who knows how to set up its BIOS? Thanks.
Optiplex 7010 turned on and off instantaneously after plugging in the power 
bar, but never pressing the power switch