I have a RadCity 5 Plus step-thru, and I was returning from my grocery run from (banally bourgeois) Trader Joes with fully loaded rear Ortlieb Panniers when I started to notice a resonant wobble throughout the bike frame when turning the front wheel even mildly rapidly. It feels quite unsafe. This occurs even when almost stopped so it isn't a speed wobble. I checked the wheels, headset and stem and everything is tight. The wobble pretty much goes away if the panniers aren't loaded, so I'm wondering if this due to the geometry of the bike and long steerer (moving too much weight off the front wheel maybe?), an insufficiently rigid fork (it feels better with the fork locked), or perhaps just a side effect of the step-through design? Has anyone else experienced this?