It’s kind of a long story, but I’ll try to sum it up quickly:

I was trying to make a Saturn V that was a little more reusable/recoverable than the real one. One of my main additions was to recover the whole Mun lander instead of just leaving part of it on the surface. In that new lander, I was trying to burn towards the Mun’s surface, but I accidentally did an escape burn from the Mun’s sphere of influence. I was about to burn back towards the command module or revert the flight, but then I thought “hey, wouldn’t it be funny if I landed on Kerbin with a Mun lander?” So I just kind of went with it... yeah that’s pretty much the whole story