I have had those shutdown explosions on every twin cylinder engine I own with the exception of the Pro-16 and 24G.
I think what happens when a shutdown explosion occurs is that unburnt fuel is dumped out the exhaust when the engine is shut off. When the ignition is shut off the engine continues to spin sucking fuel into the cylinders and out the exhaust. When air creeps in via the tailpipe is when the hot unburnt fuel ignites.
The reason that the Pro-16 is immune to this issue is likely due to the fuel solenoid on the side of the carb. As soon as the ignition is turned off, fuel is shut off inside the carb. The solenoid is a necessity on that tractor. Without it, a brief shutoff of the engine due to the operator briefly removing his hands for the OP switches would dump fuel into the exhaust. Once the operator placed his hand back on the switch, the engine fires and the flame front would ignite the fuel in the muffler, blowing the muffler apart.
For those engines that can have a fuel shutoff solenoid installed, simply adding one will resolve the issue. On carbs where a solenoid cannot be installed, the only solution is to let it idle for a minute or two permitting the hot muffler(s) to cool before turning it off. Another option would be to add a length of tailpipe. The extra length of pipe will slow the ingress of air creeping into the hot muffler.
The Kohler M18 I have is particularly prone to exhaust explosions. I am surprised the mufflers haven't blown apart by now. I am not sure why that engine is so prone to it.
The 24G seems to be immune to exhaust explosions. I believe it has to do with the length of the exhaust system. The exhaust on the 24G is fairly long and by the time the unburnt fuel reaches the muffler, it has cooled enough so that it can't ignite.
I have shut off my 24GLX after a hard run and heard two distinct pops. They were small almost humorous pops. First one popped, then the other. Since that happened, I have added tail pipes. That addition may eliminate the pops.
Fuel solenoids were an option on certain Kohler and Briggs carbs.