TVS Diodes Causing Board Failures by Failing Short, How Do I Prevent It?
I know this question is several months old but I couldn't help but respond.

I had a nearly identical problem with a project at work recently. The TVS diodes originally used on a board were too small (too low of a breakdown voltage) and would fail short-circuit, taking out the entire board. The solution for me involved sourcing a new diode, and there were two parts to this: 


Spec a diode with a breakdown voltage as high as possible while maintaining a reasonable clamping voltage (the clamping voltage must be below the absolute maximum ratings of the devices it is protecting). My project ran on a maximum of 15 volts and the chip being protected had an absolute maximum input voltage of 35 volts. I sized the diode to have a breakdown of 18 volts and a clamping voltage of 30 volts. Keep in mind that the breakdown voltage is nominal and the actual voltage may vary, so having it so close to 12 volts could be a problem. I would recommend a diode with a 15V breakdown voltage, but again, make sure the clamping voltage is lower than the absolute maximum voltage of the next device down the line. 
Make sure the diode is suitably sized to handle the power properly during a transient event (especially important during nearby lightning strikes but is still applicable for ESD). Instead of using an SMA, try using an SMB or SMC. It looks in your image like you have enough room, so shoot for an SMC.


Something else I notice, you have your reverse polarity protection (D10) going straight between the power and ground. This is very bad form as it will be shorted and damaged in the event that the polarity is reversed. You should consider using a series diode instead, or even better a MOSFET-based reverse polarity protection circuit (these are very easy to implement).