Re: flyback converters
If it is excessive dissipation, then I would carefully look at the driver circuit.
The snubber can add to the dissipation, but not too much. There are generally two snubbers: one clamps the voltage from the leakage inductance (it's an RCD circuit , from collector to +Vcc), the other slows down the voltage rise (usually just an RC, though it can be RCD, from collector to emitter). If the first fails, the transistor can be damaged by overvoltage stress, so you would not see it getting hot, it would just fail. If the snubber across the transistor fails, then yes, it is possible to increase the dissipation during transistor turn-off, causing failure due to power dissipation or secondary breakdown.
Again, I will stress the importance of the driver in this whole thing. Check the components used to drive the transistor (resistors, diode, speed-up cap).
One other thing to check is if there is a component across the current sense resistor. If that failed (low resistance), the transistor peak current will be higher, leading to failure.
If you have a scope, look at the transistor collector voltage (PAY CLOSE ATTENTION TO ISOLATION. YOU ARE WORKING ON THE AC MAINS SIDE!). If you see that towards the end of the on-time the transistor tends to go out of saturation, then very likely you have a problem with the driver.