I had something like this happen a few times this year in my syrup pan. I have an older soldered pan. We figured that after cleaning the pan there was a bit of water that worked it way into the seams and was boiling and popping a bit until it evaporated. Never made a mess like yours though.
Camp Wokanda
Peoria Park District
2023 - 210 on 3/16 shurflo, sap storage shack w/ 1100 gallon tank - 123 gallons
2022 - 210 on 3/16 shurflo, homemade vac filter & water jacket canner - 104 gallons
2021 - 215 on 3/16 shurflo, added 2nd membrane to RO - 78 gallons
2020 - 210 on 3/16 shurflo, upgraded hp pump on RO - 66 gallons
2019 - 150 on 3/16 shurflo, Deer Run 125 dolly RO - 73 gallons
2018 - 120 on 3/16 shurflo, 2x6 raised flue w/hood, homemade arch w/ AUF & AOF - 34.5 gallons