I've had a few chats with peolple in the know and have been given conflicting explanation of surge -
in the end i decided to speak to a turbo manufacturer to get the story
basically as a car comes on boost it is possible for the turbo to struggle to provide enough air that the engine is capable of consuming, the engine will kind of cough - this is evidence of surge
generally speaking - It happens on engines with a good Volumetric efficiency as they come on boost
it can also be an effect of a mismatched turbo with incorrect ex and compressor wheels