Glad to hear things are going well for your Team. The simple answer to your question is: ask the Team. Metrics only guide us in where and when to ask questions. They don't tell us much on their own.
As one of the comments mentions, velocity isn't really a measure of productivity (at least not on its own). Most measurements need to be taken with other measurements to infer anything. I'd advise also looking at the average story count in the Sprints and the average size of a story in the Sprint too. This can help you infer more about what is happening. Again though, to make sense of it, you have to talk to the Team. If they say "nothing has changed, we just get more done now" I would be suspicious.
Also, talk to the stakeholders. Are their needs getting met faster? More effectively? If so, that is really how you know your Team is doing well. More work without more value isn't really productivity.