Strange situation with stepper motor

Hello to everybody,

I have a stepper driver shield (for Arduino UNO) and Arduino Mega2560 :D
(And use GRBL for mega2560)

I connected only one motor on z-axis for my tests and this is the result.

Connect pin for DIR and STEP, connect pins for clock and strobe, connect pin for enable.
Connect Vin and ground, 12V directly on the shield and USB to arduino for the logic.

All is ok and the stepper work great.

BUT

on arduino there is a 5V pin. I read many article on internet and this pins is needed to power the logic.
If I connect the stepper going into a freeze, if disconnect work fine.

How it's possible? I break my electronic if i leave it disconnected?

Link of schematics
[www.futurashop.it]

Thanks