It would at least have been nice to say why that is though. My guess: because the support people outside expect that one first?
Passengers and crew traditionally enter from doors on the left of an aircraft. So in many situations the right engine is kept running while the left is shutdown to accommodate passenger entry. You wouldn't start left, then right, only to shutdown left to board pax.
As for safety, one would never start an engine if it would be unsafe to start a different engine. You wouldn't start the right while someone was working around the left. A single crossed wire or slip of the finger is too high a risk to take.