OK leaving aside natural price rises etc, here's how it works. I'm going to use simple low figures to illustrate what happens. Lets say without any NCB your insurance is £100, with NCB you get 60%(say) off so it's £40. Now to protect that discount you pay NCBP, what is known as re-insurance, ie your company effectively laying it off(they may not actually be using another company but the principle is the same). So if you claim you lose some of your NCB so lets say you lose half your NCB, your insurer is now charging you £70, £30 of which is paid by the re-insurer because of your NCBP. However now you have claimed you are a larger risk so your insurance will rise so now it's say £110 so that means your premium is now 70% of 110, £77 less the £30 from the NCBP policy making the new premium £47 to you, hence you perceive your insurance has risen. Add to that an annual increase and it can seem steep. The bottom line is that they will always recover a claim, never claim unless you absolutely have to.