Rejected claim - wear and tear / not a mechanical breakdown.

I recently submitted a claim to Dotsure for my motor extender policy that I took out with Hollard a few years back -> which was taken over by DotSure. They "claim" to cover a number of parts.. luckily this part was covered. It was for CV joints. I followed all the procedures and met all the requirements for my claim Full service history, registered dealer, needed docs etc... the claim was rejected as they said the part was broken because of wear and tear... and it was not a mechanical breakdown.. how can a part that is not mechanical, suffer a mechanical breakdown? Apparently it is at their discretion??? Really? So, what happens to parts with 100K KMs on the clock? When will it not be deemed wear and tear? Shocking.. they will NEVER see my business again!