I suspect you will find this is some "waffling" in the tires. This is most common to very high performance tires constructed of softer compounds to provide "stickiness" (or grip) in high speed cornering. These tires are easily identified by the wear rating imprinted on the tire.
Extreme high performance tires will have a rating in the 200-300 range. They are pretty with impressive tread patterns, but designed for sports car handling with normal air pressures. On a trike however, the combination of low air pressure (assuming you are running at a nominal 24-28 lbs) and the very soft compounds make them prone to "waffling" at 10,000 to 12,000 miles (sometime less).
On my vehicles I look for wear rating of 700-800, but on the trike I've found that a 500 wear rating gives a smooth ride with great grip and performance while providing good wear at the lower pressures.
I'm sure others will offer other opinions and tires are largely a personal preferance.
Good luck with identifying your problem and solving it.