Troubleshooting ZVS flyback failure
MOSFETs are sensitive to voltages exceeding their ratings. 

When you switch off the device, the inductive coil is very likely to have generated an inductive back-EMF of voltage high enough to damage the MOSFETs. After all, the electrical energy stored in an inductor has to get out of somewhere - When you power off, the energy finds some path or the other to discharge itself, and resultant voltages can be considerably higher than the original voltage across the inductor. 

A generic fix for inductor back-EMF woes is to apply a reversed diode across the coil, or in this case, possibly across the MOSFETs. Do not depend on the body diode of the MOSFETs to provide a discharge path, as body diodes rarely have suitable characteristics to serve as a high energy discharge path.