Suspect a "tired" ship - normal short sea trading for the first 15 years (she started life with Carebeka in N Holland) but then the last 20 in the dirty rough trades with granite, limestone etc, mainly on very short runs and so very frequent loading/unloading. Not very forgiving for a ship's structure. Very easy for unidentified problems to be lurking, waiting for that exceptional storm when carrying something like 3000 tons of rock.
But some indicators are there. Change of Class in 2009 from Lloyd's Register to International Naval Surveys Bureau (IACS and high performing to non-IACS and medium-performing, according to Paris MOU). Change of registry from Barbados (white list) to Cook Islands (grey list) in the same year. One of the consequences of choosing class and flagging further down the list is more frequent flag state inspections - in the case of SWANLAND two-to-four each year - 35 over the last decade. In all those she was detained only once, for a couple of days back in 2003 at New Ross. Usually a few defects found each time, but I would think less than average for a vessel of her type and age.
That said, in inspections over the last few years there have been more frequent instances of deck corrosion and minor cracks, including at an inspection at Shoreham in May. However, she had an MCA "detailed examination" at Sheerness in October with almost clean bill of health and no structural issues at all. Her five-year survey not due until 3/2012.
(I wrote the above for another site in 11/2011, but it seems still relevant).
No doubt the MAIB are paying particular attention to those two recent MCA inspections. On the face of it, neither of them justified detaining the vessel - the authorities cannot just "arrest" (actually "detain") a vessel because it has a mediocre history, there has to be a technical judgment of "unseaworthiness" at the time. It seems that the owners/managers have always managed to do enough immediate rectification - a bit of welding here, an extra stiffener there - to satisfy the port state inspectors' requirements.