Powering arduino and sensors and motor damaging 16x2 LCD?
In some instances, it is a known issue that certain LCD's pull more power from the arduino than is sustainable depending on the power source. 

A few suggestions:

Put something like a 100-200 ohm resister in the 5V to the LCD and connect just that and GND, then measure the voltage drop across said resister.  That will give you the current with ohms law, if its much more than 4mA its a faulty LCD or a fault in your wiring. 

You should also check the pins for continuity errors, or shorts. 

Another idea, is to make sure the voltage, and amperage coming from the 5vdc step down you are using is adequate for the task at hand.

And finally, you'll need to calculate the complete pull on your power source. i.e. you'll need to know how much load your bigeasy driver pulling from the 12vdc power supply, as well as how much your 5v stepdown is using. if they are exceeding the output of the PSU, you can run into issues. 

I ran into a similar issue with my own project, using an arduino Uno with grbl + 3x easydriver motor controllers, &amp; mega 2560 + lcdkeypad shield. Powering the easydrivers with 12vdc &amp; using a voltage stepdown module also gave me issues, as it didn't provide enough current for the stepper motor drivers and the LCD + mega + uno from the 5vdc. I was overloading the power supply, and the 5v wasn't producing the correct power requirements. 

I bought a better power supply that had 24, 12, 7, and 5v on it, and haven't had an issue since.