Sudden turbo overboost

Returning home from seaplane fly in in the Adirondack's no and heard a pop and suddenly the mp went over boost and retarded throttle to keep mp below 39 but rpm was only as high as 3900 which wasn’t enough to maintain altitude. So luck would have an airport within powered glide. Upon ground turbo cable goes through normal self test. So not sure what is causing the sudden loss of proper turbo control. It looks like if I hit the turbo disconnect switch at the right time during the initial cycling I should be able to get home. Any thoughts troubleshooting this problem