When the USMC first began operating the AV-8A, the first sqn formed was filled with qualified test pilots, who had an excellent safety record and reported the Harrier was very easy to fly. Obviously for test pilots it presented no problems because they are of a much higher standard than regular sqn pilots. After this initial period, the USMC authorities that the Harrier would present no difficulties, and went a step further; as th Harrier could land and take off vertically and hover, then why not post helicopter pilots to the Harrier sqns? This they did, and the former helo pilots had no problems in the vertical, they just had a bad habit of flying into hills and mountains at 500 knots. The pilots just didn't have the 'currency', as the Marines called it, for fast jet operations. This is the USMCs own explanation for the high accident rate in the 70s and early 80s.
Since the introduction of the AV-8B the problem has been partly down to insufficient flying hours. When one aircraft crashes, the whole fleet is grounded and the pilots begin to lose proficiencey if the grounding drags on. When the fleet returns to service the pilots are less practiced than they should be and the liklihood of accidents increases, again this is from the USMC itself (though not necessarily the top brass). Also remember, when embarked aboard the LHAs/ LHDs of the USN, the Harriers are effectively 'second class citizens' as helo ops take priority. When a USMC sqn embarked in HMS Illustrious last year the contrast was very dramatic from their point of view, and they found it very refreshing to be on a ship that put them first.