Metal fatique is always a possibility with an old plane, and corrosion is a concern for planes operated off of salt water. That said, the Grumman Mallard is a very strong aircraft (Grumman's nickname was the Grumman Iron Works because their planes were always hell bent for stout). The plane was reengines with Pratt & Whitney (Canada) PT6 engines, a design with a long and highly successful track record. PT6 engines have been in service for more than 40 years.

At this point, it's hard to speculate because it could've been any number of things. The reports of an engine explosion (if accurate) could possibly be an indication of turbine blade failure. Such a failure could've launched very hot pieces of metal at very high speeds, possibly puncturing a fuel tank. There was an FAA service bulletin earlier this year to inspect and repair as necessary the propellor bolts. Shedding a prop under takeoff or climb conditions (when the engines are operating at the highest power settings) could be very unhealthy.