Panicum is a warm season grass, meaning that it doesn't really start growing until weather warms sufficiently in late spring or early summer. I'd suspect that when you purchased them last spring, they had been encouraged into earlier than typical growth by being grown in a greenhouse and this season they are just following their more normal growth habit.

You are correct in assuming the laxity you noticed being due to a richer planting mix. US native ornamental grasses tend to flourish in rather poor soils and rich soils tend to encourage rapid, lush growth that can result in floppier plants.

Thank you, the thought on the first season makes sense they probably had been in poly tunnels, so warmed and ready to grow. Though more established plants in another garden were up and away, possibly a combination of time in the ground and local temperatures.