As people have said already this is a somewhat common failure mode, especially when swapping nozzles. This happened to me twice between three nozzle swaps. The first time was a major leak like yours, the other time it was only slight (which I then made worse in my attempt to fix…). I was obviously doing something wrong, but I came fully prepared the second time with video guides for my specific machine and everything but still couldn’t get it perfect.

If you never want to think about this failure mode again (like me) then consider swapping your hotend for a Revo. A Revo nozzle is also the heatbreak, so there’s no possibility of a bad connection between them. The ‘nozzles’ are more expensive but they can be hotswapped (coldswapped, even) by hand with no special tools. Before I did everything in my power to avoid nozzle swaps, so I ended up settling for a jack-of-all-trades (but master of none) nozzle that I would never have to swap. Since moving to Revo however I find myself swapping nozzles way more now that it’s easy and with no chance of destroying my hotend. For instance I have a high-flow 1mm nozzle for quickly doing big structural prints, they print in like one third of the time and are way stronger than equivalent prints on a smaller nozzle. I also have a 0.25mm nozzle for miniature model prints with a better resolution than I could ever get before. I’m still waiting for a high-flow abrasion-resistant Revo nozzle, though.