Take your big wallet to the parts shop & buy a new one or pull the plunger up as high as you can and cut it off.
Then when you finish mowing push the throttle down to the minimum count to 10 and turn off the engine .
If you don't do this the unburned fuel will accumulate in the muffler and ignite once the air:fuel ratio is right for combustion at atmospheric pressure.
Then it ignites from a still red hot muffler internal & goes bang, often a minute or so after you have gotten off .
It does not hurt the mower or muffler . just sounds bad
Side note in some places chopping the plunger off can leave you open to very big fines ( $ 5,000 to $ 10,000 ) so if there is an environmental idiot around your place or you are having a fued with your neighbours not shutting down quietly can cost you big time .