As he proceeded northeast in the fading daylight, he would have depended on the emerging lights of the east coast to maintain his visual flight reference and fly the aircraft safely. It would have been completely dark by the time he reached abeam Martha's Vineyard. His turn out to sea toward the island would have put the lights of the east coast behind him. He would have been turning into the black abyss of the Atlantic, leaving him with no visual reference by which to fly the aircraft properly. W ithout visual reference, the inner ear can play tricks on you.
Every aviator knows about vertigo.
You fly into the clouds and lose visual reference to the horizon. Mr. Kennedy probably got vertigo and put the aircraft in a bank because that was what his inner was telling him he needed to do to keep straight and level. As he banked, the nose begin to fall through, placing the aircraft in a powered descent. He might have been puzzled by the increased slipstream noise from the gaining airspeed. But without experience and training it would have been hard to recognize the need for prompt corrective action.