That sounds like what is called a "pilot chute hesitation", where, as you said, the pilot chute (drogue chute in this case) gets into the still air behind the jumpers. It's a minor problem, especially with a tandem where the drogue is deployed right out the door. There's all kinds of time available to clear it, and it usually does so itself in just a second or two. For experienced jumpers who deploy much lower, and don't have as much time to fix things, it's a bit scarier. But for a tandem, it's nothing much for a tandem passenger to really worry about. Sounds like the tandem master handled it just fine.
