It was likely a loose nozzle that wasn’t properly heat tightened.
For those who don’t know, after screwing the nozzle in cold, crank the hotend up to max (or hotter than you ever plan to print by about 10C) and tighten the nozzle again. The thermal expansion is different between an aluminum heat block and a brass nozzle. The extra tightening while hot can overcome this problem.
It was likely a loose nozzle that wasn’t properly heat tightened.
For those who don’t know, after screwing the nozzle in cold, crank the hotend up to max (or hotter than you ever plan to print by about 10C) and tighten the nozzle again. The thermal expansion is different between an aluminum heat block and a brass nozzle. The extra tightening while hot can overcome this problem.