In contracts management, what are the causes or basis of the "change order claim or variation order"? any thoughts?

It could be change in local governing laws in the country or local market inflations which increases the overhead and resource cost to the company which in turn pulls down the profitability of the contract and prompts for the need of a change order claim or variation order. It could be even a variation to the agreed contract scope. It all depends upon the nature and terms and conditions  of the contract.

Project change order or variation is different than the contract change orders. Project change order is specific to that particular project alone.But the contract change order may imact the multiple projects governed under that particular contract in various aspects.