Bonnet explosion on Statesman V6

It's not unusual to have explosions like you experienced, in fact   it's relatively common. The explosion occurs when the air/gas mixture in the intake manifold ignites and the rapid expansion of the gas mixture often blows out the air cleaner like it did on your car.   Ignition is usually caused by a random spark, so you have to find and fix the cause of that spark. The mechanic was on the mark when he suggested you change the spark plugs and clean the throttle body, and I would have changed the spark plug leads as well. His advice that it would settle down is wrong, it will continue to occur until the cause is identified and fixed. He's probably hoping that you will have left town by the time you realize his advice in this case is incorrect.