SCR in this circuit exploded - why?
The only sure limitation of the current is your common-mode choke and your internal wiring. 

The S6020L (if your SCR really matches the specifications) is capable of a one-time pulse of 0.01second and 255A at room temperature. So if your wiring + CM choke is less than around an ohm (and it will not be), there is no guarantee you won't exceed the maximum specifications (and it should be noted that you really should stay reasonably far away from these one-time pulse at room temperature limits- it may be hotter than room temperature and repetitive surges may kill the SCR eventually). 



Since you have a sync signal, and presumably a processor reading that, I think the best solution might be a firmware change that triggers the SCR only at the zero crossing. The current would then be limited by the dv/dt of the mains line at the zero crossing. At 220V RMS, and \$\omega\$ = 2\$\pi\$50 = about 0.1V/usec, so the current will be limited to 56A, well within specifications. 

Failing that, perhaps you could change the opto-triac for one that has built-in zero crossing detection (eg. MOC3063), but I have not looked carefully to see if that will work with your diode setup- you may need to put a resistor across C1, for example.