Roper lawn mower stopped working, blown spark plug

Hello everybody! I just purchased a second hand lawn tractor, it has a 14hp briggs and stratton vanguard engine.
 
It was running fine, just a bit high revving at idle. I changed the oil and filters, and just rode on it a bit. After 5 minutes, it started knocking and stopped, and it won't start again. Now when I turn the flywheel, it hang in something, and I removed a spark plug, it seems the electrode exploded inside the cylinder. Do you know what could cause this? I live in Romania, and these motors are quite expensive, but parts are readily available. Do you have any insight on what could have happened?
 
Here is an image of the spark plug, and a video of what is happening: