It's past simple. 'Changes' in this sentence is a noun not a verb. A similar sentence: Who broke the glass? The culprit was the cat. The full noun phrase is: 'changes to its underlying internet infrastructure that coordinates the traffic between its data centers.'

It's not structure per se. But more like the substructural technology they've got by which their data centres are able to send and and receive information. as for "the culprit", when someone or something is at fault or responsible for a problem or any unpleasant occuring for that matter, they're call culprit.