lelebebbel - my guess would be the tip vortices interfering with the following blade.
When they created the BERP blade for Lynx and subsequently EH101, the first iterations had problems with what was referred to as 'cobblestoning' in the hover - this was mostly alleviated by adding an anhedral tip to help shed the tip vortices downwards.
The more blades you have the more vortices you are producing.
You could also factor in the blade passing frequency over the cockpit and tail boom as a source of pressure pulses that could affect structural integrity if they match the resonant frequency of the airframe - I believe this is a problem on the S92 creating higher noise levels in the cockpit.