What are the reasons for growing case laws concerning NEC3 ECC in recent times. Could it be because NEC3 is the most widely used suite of construction contracts?

You seem to have asked this question before which we have answered but I will answer it again with the same answer:

I am not actually sure that they are, but IF they are, I think it has gone from zero cases to a few cases. That is a rise - but no doubt an inevitable one given the amount of NEC3 contracts out there and the value of work that these are now being used for within the industry, but still very few.

The good news is that any such rise as still very nominal - and certainly much less than other form of contract that continue to end up in court (in my experience) much more than NEC ones.