Hi, I do not think this is caused by your technique. It could be a mechanical problem or out of spec bike. There are many threads on this but the frame wobble seems to occur with larger riders and large frame sizes because of the geometry of those frames. But..... a bike should not wobble at only 30mph. I would take this bike back to where you purchased and have them check it out. Look for fork geometry and whether it is in manufacturing specs., and make sure the front wheel is correctly seated in the drop outs. I do not know about Kestral being any more prone to this problem than other brands but I would get my money back if it cannot go 30mph on a downhill without the frame shaking.

Defensive action. If this occurs jam your knee into the top tube to break the oscillation pattern and stay clipped in and on the drops initiate braking.