Skippers - the first few pages of the report Gordy links to start to give some reasons - the 'very responsive' rotor system, the unusual head design that allows the blades to flap independently and the unusual cyclic setup are all contributory factors.
Add in a great many inexperienced pilots flying the aircraft and you have a worrying mixture.
The stats for LOC (loss of control) accidents per flying hour are significantly higher for R22.
It does seem that flying too fast in a robbie can trigger events that result in similar outcomes to low G and turbulent conditions - makes me wonder if an excursion into retreating blade stall might prompt an inappropriate control response leading to the mast-bumping, tail strike and rotor separation sequence we are familiar with.